Guide to defeating doppelganger

The following tactics have been suggested to beat the Doppelganger:

General

 * The Doppelganger will have the same skills you equip, so don't equip any skill you wouldn't want used against you.


 * Think very carefully before bringing any skill that can be used to heal yourself.


 * The AI generally prioritizes skills to the left of the skill bar.


 * Equip lots of slow-casting self-buffs: the less useful in the one-on-one fight, the better. The Doppelganger will waste time at the beginning of the fight casting every buff on itself.


 * Cast any self-buff enchantments before stepping on the third stone that starts the cut scene; they will last into the match. Cut scenes no longer run in real time, so any enchantments with a duration will not expire or run down their time during the cut scene.


 * Dodging arrows: Stand as far away from the Doppelganger as you can be and still cast spells on it, and strafe left and right as fast as you can. Make sure to constantly tap both ways. This twitchy movement will cause the Doppelganger to shoot wide and miss most of its shots.

Hard Mode Notes


 * Consider switching to a Necromancer secondary and using the following setup. Equip Enfeeble, Insidious Parasite, and Spiteful Spirit and max out your Curses attribute to 12. This will cause the doppelganger to kill himself relatively quickly, granting the completion of the Bonus objective. The rest of the skill bar is not important and should be left empty to increase the amount of time the doppelganger spends attacking; do not attack him yourself. This strategy works for any profession with the advantage that the three spells are all from the Prophecies campaign.

Elementalist

 * Bring lots of skills with very long cast times, and only one or two spammable skills. The Doppelganger will usually focus on the slow-casting skills and allow you to gain the advantage by wise use of the spammable skills. This is not foolproof, but the AI tends to use all of its skills relatively evenly, whereas a human is smart enough to focus on different skills at different times.


 * A successful example of the above strategy is to use Fire Magic and bring Fire Attunement, Glyph of Elemental Power, and Flare, along with several other useless skills such as Air, Water, and Earth attunements. Before stepping on the 3rd stone to enter the battle with the Doppelganger, cast Fire Attunement and use Glyph of Elemental Power.  Then enter the fight and spam Flare.  The Doppelganger wastes so much time casting attunements and preparing itself that is it possible to win the fight in 10-15 seconds or so, sometimes without taking a single attack from the Doppelganger.


 * Step out of AoE DoT spells like Meteor Shower or Maelstrom; the Doppelganger will not.


 * Do not equip skills that could disrupt your spell casting (like Gale or Earthquake).


 * Blinding Flash


 * If your are E/Me, consider bring Physical Resistance with low levels of Inspiration Magic but do not use it yourself, allow the Doppleganger to use it to increase your damage output.


 * You can just take flare,water attunement,fire attunement some stances that reduces physical damage and fireball, He will use the stances quickly so spam flare while he is wasting time with stances and attunement and by the time he is finished he is almost dead. Fireball occasionally to make the battle shorter.

Hard Mode Notes


 * Pre-cast Glyph of Sacrifice on Savannah Heat and bring Searing Heat, Liquid Flame, Fireball, ect.


 * Using an Echo nuker Build with Mark of Rodgort also works fairly well, so long as you dodge his Meteor Showers. Bring Fire Storm too for extra damage. He dodges them occasionally, so it's better to cast Fire Storm and Mark of Rodgort right when he tries to cast Meteor Shower, then follow up with the nuking.


 * For both modes, go E/Me, take only Empathy and then Blinding Flash, Blinding Surge or some other anti-mellee skills to prevent damage to oneself, and do nothing (except from re-applying blind and Empathy). The Doppelganger will attack you, dealing damage to himself until he dies.

Mesmer

 * Put lots of useless, spammable spells to the left and Backfire on the far right. If you cast Backfire in the first few seconds the Doppelganger usually dies in about 6 seconds. Usually, only 2 skills are needed: Backfire and Wastrel's Worry, with all of your attribute points in Domination Magic and Fast Casting. Cast Backfire immediately, then use only normal attacks. This method has been proven to work at level 11, with a Superior Rune of Domination. A monk secondary with Guardian and Empathy works well for the ending part; Guild Wars AI prevents a monster from killing itself with backfire.


 * Bring Distortion, Empathy, and Spirit of Failure. Keep the Doppelganger constantly hexed while always keeping yourself under Distortion and it should attack until it dies. Signet of Midnight works as well if you have it. Also, try to keep in its melee range so it will have a higher attack rate. Any secondary profession skill which increases attack speed will help as well.


 * Illusion of Weakness can be very effective. Cast it and let your health regenerate before entering the battle. The Doppelganger will use it during battle, lowering its health and giving you an advantage. Further take advantage of this by only using degeneration hexes, as they will not trigger its Illusion of Weakness to heal it. Also, have it near the right side of the skill bar, as it could be a fitting finishing blow.


 * Illusion of Haste is often a good idea. The Doppelganger will waste time casting it (and give itself an enchantment you can use Shatter Enchantment on), but since neither of you is likely to be moving it will be of little use to the Doppelganger.


 * Be careful with interrupts. The AI almost always has faster reflexes than you, so you'll get interrupted more than it will. Only bring interrupts if you have also equipped a skill with a very lengthy cast time, in which case let the AI use this skill, ignoring using it yourself.


 * Another good combination that can work with the above is Spirit Shackles, Mind Wrack, and Empathy. Bring Ether Feast to heal yourself. Immediately cast Spirit Shackles and once the Doppelganger has reached negligible energy (less than 5) continually cast Mind Wrack, and occasionally Spirit Shackles and Empathy. If needed, bring Physical Resistance as well (most easily cast just before the cut scene). Domination Magic should be as high as possible, Inspiration Magic at around 10, and Fast Casting at whatever is possible with the remaining points. Note: DO NOT attack with your weapon, as you'll have all of the same hexes on yourself. If you have a problem killing the Doppelganger in under 50 seconds, also bring Arcane Echo and use it with Mind Wrack once its energy is low.


 * Only bring Clumsiness and remove your weapon. Then simply spam it on your doppelganger and you will win quite easily.

Hard Mode Notes


 * The AI appears to not use spells when it is at low health and has Backfire on it.


 * For all modes, take only Empathy, cast it on the Doppelganger and do nothing. He'll attack you and kill himself. One could also go Me/E and use Blinding Flash, Blinding Surge or some other anti-mellee skills to prevent damage to oneself. Equipping Signet of Midnight and Frenzy will hasten its death at no danger to you.


 * Only bring Clumsiness and remove your weapon. Then simply spam it on your doppelganger and you will win easy enough. Note: put as much as you can into illusion attribute to make it quicker, with 16 it takes less than 40 secs to win.

Monk

 * Consider bringing Healing Breeze. The AI currently will not use Healing Breeze for ordinary healing. It does, however, use Healing Breeze to counter health degeneration from conditions or hexes. If you are a Monk/Mesmer, bring Empathy and Healing Breeze. Simply cast Empathy on the Doppelganger, keeping Healing Breeze up on yourself.


 * Bring useless maintained enchantments. If you bring at least four, then the Doppelganger will cast them all on itself and have no energy regeneration (or even degeneration if you have more than four). Divine Boon will cost it extra energy when it heals; when it runs out of energy it will stop healing and have no benefit. Life Attunement will make it deal less damage; since it cannot heal due to lack of energy it gets no benefit. Blessed Aura is good too, since it cannot cast enchantments due to the aforementioned lack of energy. Holy Veil is good if you don't plan to use hexes. Protective Bond can be useful in addition, if you cast it on yourself. It prevents you from taking too much damage at the cost of taking your energy, but is not as useful to the doppelganger as it is already losing so much energy.


 * Scourge Healing.


 * If you are a Monk/Necromancer, bring Scourge Sacrifice, Awaken the Blood, and Order of the Vampire. No other skills are needed, merely cast Scourge Sacrifice on the Doppelganger and wait for it to kill itself. This method works well in hard mode as well. Additionally taking Blood Renewal as well will kill him even faster as he only uses it after taking damage (from Order of the Vampire).


 * If you are a Monk/Necromancer, bring useless Monk enchantment spells and Rend Enchantments. Cast the enchantments on yourself; the Doppelganger will lose over 200 health when it casts Rend Enchantments. Also bring other skills that require it to sacrifice health, such as Defile Flesh and Dark Pact. Alternatively, bring the elite Corrupt Enchantment with the enchantment spells; two or three casts of Corrupt Enchantment when the Doppelganger is enchanted should take it down in 15 seconds.


 * You might want to use Protective Spirit to counter the Doppelganger's attacks. The AI will cast this spell on itself, however it will most likely have no effect as your own attacks are unlikely to exceed 10% of the Doppelganger's health.


 * Bring Retribution, Protective Spirit, and Healing Breeze. Precast Retribution, and heal using Healing Breeze while maintaining Protective Spirit. This strategy can work for even very low level characters.

Hard Mode Notes


 * If you do a 55 or 105 build with Spoil Victor you can kill the Doppelganger in Hard Mode. Just use a regular 55/105 build and use Reversal of Fortune to heal yourself instead of using Healing Breeze.  Just move up close enough so he can attack you with his sword and let him kill himself.
 * The Hard Mode AI will use Healing Breeze.
 * Replace Healing Breeze with Restful Breeze. Doppelganger constantly attacks/casts so he won't get any benefit from it.

Necromancer

 * Bring spells that require a health sacrifice. Don't use these skills yourself, but rather let the Doppelganger use them. By including Awaken the Blood and/or Cultist's Fervor as the first skills in your skill bar, the Doppelganger will use them first, thereby increasing the health sacrifice. The elite skill Order of the Vampire is a very good choice for this role. With a 17% max health sacrifice and a 5 second cooldown, the Doppelganger is likely to kill himself in seconds. A Necromancer/Monk may also wish to bring Scourge Sacrifice in order to further increase the health sacrifice.


 * Take skills that inflict conditions upon yourself, such as Signet of Agony and Chilblains. Don't use them yourself, let the Doppelganger use them, giving itself conditions such as poison and bleeding while you only take a small amount of damage.


 * Insidious Parasite alone, with curses at 12 and no special weapon will heal you and damage the Doppelganger at the same time. This skill by itself with no special weapons or preparations can kill the Doppelganger in under 30 seconds, while offsetting nearly all the damage you take.  Note that the Doppelganger will also use this skill, so do not attack.  Just keep applying this curse and wait for victory + bonus. SS and Reckless Haste/Price of Failure can be used similarly.


 * If you are a Necromancer/Monk, bring useless Monk enchantment spells and Rend Enchantments. Cast the enchantments on yourself; the Doppelganger will lose over 200 health when it casts Rend Enchantments. Also bring other skills that require it to sacrifice health, such as Defile Flesh and Dark Pact. Alternatively, bring Desecrate Enchantments and Defile Enchantments, letting the Doppelganger use the enchantment spells. Note that both of you will be enchanted by Divine Fire, thus Corrupt Enchantment alone will not guarantee an easy win (but an easy bonus if you do win).


 * Consider bringing Soul Barbs along with useless enchantments such as Death Nova to cause the Doppelganger to damage itself.


 * Don't bring life steal skills or hexes such as Life Siphon and Life Transfer. The Doppelganger will likely also cast them on you, so at best it will be an even trade and cancel out.


 * Remember that there will be nothing dying in this battle, so not only will Soul Reaping be worthless, anything that requires a corpse (such as any well spells, any minions, Soul Feast, Consume Corpse, Necrotic Traversal, etc.) will do you no good.


 * Bringing a hex and Necrosis can help, as the doppelganger will be unable to cast Necrosis, a sunspear skill.

Hard Mode Notes


 * Switch to a Assassin secondary and bring Shadow Form, Expunge Enchantments and Dark Pact with maxed out Blood Magic, 12 Deadly Arts and 0 Shadow Arts. When the battle begins wait for the Doppelganger to use Shadow Form, expunge it, and wand him until Dark Pact recharges, then use it to kill his remaining HP or he can kill himself while using Dark Pact.
 * Another way is to bring a build with Enfeeble, Insidious Parasite, and Spiteful Spirit on it, with 16 Curses. Fill the rest of the bar with useless skills.  When you get in, immediately hex the Doppelganger with both SS and Insidious and put on Enfeeble, then stand there, only renewing the hexes when they're about to run out.  The Doppelganger will spam Enfeeble while attacking you with a bow, killing the Doppelganger in about 30 seconds.  With Weakness and Insidious on the Doppelganger, any hits from the bow will heal you more than they hurt.
 * Another way of doing this, which is a little more tricky, is to equip as many sacrifice skills as possible, and Price of Failure. Cast Price of Failure once you get into the arena, then back off a little and run from the right to left and try to dodge his spears. Inbetween shooting at you he will use the sacrifice skills to lower his health faster. Don't be afraid to give him Barbed Signet as it will not kill you, only damage you. Be sure to put as much as possible in curses while using this technique.


 * For all modes, take only Insidious Parasite, cast it on the Doppelganger and do nothing, he'll attack you and kill himself.

Ranger

 * Bring many slow-casting Nature Rituals like Winter and Quickening Zephyr (bringing about 5 to 6 Nature Rituals should enable you to kill the Doppelganger without attack skills before it can even start to attack, if you have a maximum damage bow. Bring Favorable Winds and put it at skill slot number 1, as the Doppelganger will use it first in that instance, thus strengthening your attacks and increasing your attack speed while it casts more useless Nature Rituals). The Doppelganger will waste time casting the rituals while you attack. If you are R/Mo, bring some smiters like Balthazar's aura


 * Equip bow attacks and stay within melee range of the Doppelganger; it will switch to its axe and thus be unable to use the bow attacks.


 * Bring traps and high Wilderness Survival. If you manage to have the Doppelganger set off a trap or two (use a defensive stance to improve the odds you won't be interrupted) it can give you a significant advantage.


 * Alternatively, get your pet to do all the damage. Set Marksmanship to zero, and Beast Mastery and Expertise to max. Then, take the skill Enraged Lunge as your major skill, and a few pet buffs and shouts, ie. Symbiotic Bond, Call of Protection, etc. Predatory Bond should also be taken, as it means you have healing and the Doppelganger does not. As long as all of your skills are pet based, the Doppelganger is useless. Simply instruct your pet to attack, run behind a wall where the Doppelganger cannot hit you with his bow, and use pet healing and attack skills. This tactic works in Hard Mode, even with a low level pet and ranger as a secondary profession, provided you have enough energy to keep up the attacks.

Hard Mode Notes


 * It appears that it is in the doppelganger's priority, to cast all subsequent nature rituals first, and then later attack. One is able to take advantage of this, by equipping up to five long-casting Nature Rituals, Burning Arrow, and Apply Poison. Simply keep up Apply Poison, and spam Burning Arrow while the doppelganger is spamming spirits for a rather quick victory.


 * The Nature Rituals strategy can be combined with the pet strategy described above. By bringing a few nature rituals as well as a pet (using enraged lunge as the elite skill along with a few few shouts and pet attacks) a large portion of the doppelganger's life can be removed before it even starts attacking the player.  In addition, if disrupting lunge is used to interrupt one of the nature rituals, the Doppleganger may begin attacking the pet instead of the player.

Warrior

 * Equip Frenzy; but don't use it. The Doppelganger will, putting itself at a significant disadvantage. Couple this with a few energy-based attacks which you can use right away (such as Power Attack etc) and the Doppelganger should go down in 15 seconds or less.


 * The Doppelganger does not use a shield, so bringing skills that require a shield (such as Shield Stance and Shield Bash) gives a significant advantage.


 * Equip an attack combo from left to right in reverse order (such as Final Thrust, Gash, Sever Artery). The Doppelganger doesn't understand the order certain skills should be used in, and prioritizes them from left to right on the bar when they are charged. (This strategy does not work in Hard Mode)


 * Another very useful hint is to only equip the Eye of the North skill Ursan Blessing. Using this skill will defeat your Doppelganger in less than 10 seconds.


 * Any skills that cause you to lose all adrenaline (such as Wild Blow) can be a great benefit. Ignore them yourself and let the Doppelganger waste its own adrenaline using them on you.

Hard Mode Notes


 * You can actually easily kill the Doppelganger by not even attacking him. One method is to use Signet of Stamina, Dolyak Signet, Gladiator's Defense and Deadly Riposte. Max your attribute points on Strength and Tactics only.  When you first enter the mission active Signet of Stamina and proceed toward the Doppelganger.  Get close to him so he starts using his Axe and activate Gladiators Defense and spam Deadly Riposte. When Gladiators Defense is ending use Dolyak Signet.  Repeat until he's dead.


 * He doesn't seem to use Frenzy even when it's first in your skill bar.

Ritualist

 * Equip Soothing, Shelter, Shadowsong, Pain, Bloodsong, and Dissonance with your Communing attribute as high as possible. As soon as the fight begins start summoning spirits, starting with Dissonance (to keep the Doppelganger from summoning the spirits itself) and going backward in the list. You don't need Soothing and Shelter: they are only equipped to keep the Doppelganger busy and reduce the chance that it also starts with Dissonance. Typically it will be dead within 25 seconds.


 * Equip only Vengeful Weapon and Weapon of Remedy with high Restoration Magic. Repeatedly spam on yourself for an easy win.


 * Equip useless spirits (and don't plan to use them yourself) such as Disenchantment, Soothing and Recovery, these will have no effect on you, and will waste the Doppelganger's time. Use a high Channeling Magic attribute with Renewing Surge, Channeled Strike any item spell that you can hold for awhile like Cruel Was Daoshen and if you can use Caretaker's Charge Just hold the item and spam Caretakers charge, renewing surge and channeled strike. The Doppelganger should go down like butter, considering you can do this while he's busy casting the spirits. This can kill him in as little as 6 seconds. You could also bring along Gaze of Fury to make his spirits add to the damage, he can not use his own Gaze of Fury on your spirit of fury.


 * On Normal Mode, he can be killed with only Vengeful Weapon and Spirit Rift, both being skills you receive at the beginning of the game.

Hard Mode Notes


 * You can beat your Doppelganger easily by switching to a Mesmer secondary and using Vengeful Was Khanhei and Vengeful Weapon. By using Arcane Echo to maintain Vengeful was Khanhei while also using Vengeful Weapon whenever possible, the Doppelganger will go down in a little over a minute. No other skills are needed besides Arcane Echo, Vengeful Weapon, and the elite Vengeful was Khanhei.

Assassin

 * Bring a pair of vampiric daggers and no skills with high Critical Strikes and Dagger Mastery.


 * Don't take any Dual Attacks, but include hexes that trigger on them to have the Doppelganger waste its time and energy casting them.


 * As a general rule, if you can start a dagger chain that you can use but the Doppelganger will not, it is quite easy to out damage it. Falling Spider and Horns of the Ox make a great combination that can be alternated to keep an ongoing chain of damage and degeneration on the Doppelganger. By utilizing methods to knock down the Doppelganger (allowing Falling Spider) that it will be unable or unlikely to use, you can start the chain without worry of retaliation. An example of this is to be an Assassin/Ranger and bring a pet. All you need is a pet attack to cause a condition (such as Poisonous Bite), follow with Iron Palm, and now you have the knockdown to start the chain.


 * The Unsuspecting Strike, Wild Strike, Horns of the Ox, Falling Spider and Twisting Fangs attack chain with high Critical Strikes and Dagger Mastery attributes can defeat the Doppelganger in about 5 or 6 seconds, provided you get off the combo first. To keep the Doppelganger occupied, bring along a slow-casting, useless skill and put it at the far left of the skill bar. Use Death's Charge to shadow step to it at the beginning and attack.


 * Bring Shadow Form and Expunge Enchantments as your only skills while having high Dagger Mastery and Critical Strikes. Don't use Shadow Form, instead let the Doppelganger use it. Expunge it and launch your attacks for instant win.


 * Bring Shadow Form with highest shadow arts and a set of 7 attacks. Do bring Horns of the Ox, Falling Spider, Death Blossom. You can also bring a offhand attack that needs enchantments, one lead and another offhand. The Doppelganger will be dead at the end of the sequence most probably (around 8 secs).


 * You can also try the 'A/Me Solo Sin' Build by bringing Arcane Echo, Shadow Form, Empathy, Black Lotus Strike, Black Spider Strike and Twisting Fangs. To see a video of this in action, click here.


 * Equip Shadow Walk and shorter-duration enchantments that can keep you alive, like Shadow Refuge. The Doppelganger will shadow step to you, but be unable to cast any enchantments for 30 seconds. Follow up by laying into it with a dagger combo starting with Golden Phoenix Strike to kill it. Make sure not to bring any other stances.


 * Equip from left to right in this order Frenzy, Recall, Critical Agility, Golden Lotus Strike, Fox Fangs, Twisting Fangs, Shadow Refuge. Apply Critical Agility before stepping on the last platform and when the fight starts, go for broke with Golden -> Fox Fangs -> Twisting Fangs...a couple seconds later you're done with bonus included.


 * You can simply beat this by having the basic shadow prison spiking build. Skills: Shadow Prison, Tiger Stance, Black Lotus Strike, Twisting Fangs, Black Spider Strike, Blades of Steel, and include a Malicious Strike in there if you don't think that is enough.

Hard Mode Notes


 * In Hard Mode, Empathy, Shadow Form, and Feigned Neutrality work well to have the Doppelganger kill itself. The 'A/Me Solo Sin' build is still quite effective.


 * In Hard Mode the second to last strategy above also works if you time it right.

Paragon

 * Use a lot of Spear Attacks, and stay in melee range. The Doppelganger will use its axe, and will not be able to use these attack skills.


 * Equip chants that have an effect based on spell casting, such as Aria of Zeal or Aria of Restoration. The Doppelganger will waste time and energy using these for no benefit.


 * With a Paragon/Elementalist this was successfully completed in under 25 seconds. Attribute allocation is not of major concern providing sufficient points for weapon and shield are maintained. Place no attribute points into the secondary profession line. Allocate a couple of large energy requirement spells or skills in the first 2 skill slots; attunements like Air Attunement and Fire Attunement are ideal, using 10 energy each. The Doppelganger will always apply these first. In your final skill slot place Aggressive Refrain and apply this echo while the Doppelganger is casting the attunements. Avoid skills that increase defense such as the elite "Incoming!" or "Stand Your Ground!". Attack skills that are effective include Barbed Spear and Blazing Spear, enhanced by "Go for the Eyes!". Once your echo has been applied simply attack and use adrenal skills when charged.


 * Bring Aggressive Refrain, Anthem of Flame, Signet of Aggression, and adrenal spear attacks.

Dervish

 * Bring Vow of Strength with high Scythe Mastery and Earth Prayers. Cast Vow of Strength after the cut scene and attack the Doppelganger; it will go down in less than 20 seconds. A Dervish/Warrior can bring Frenzy for an even easier kill. A vampiric scythe with +15% damage while enchanted also helps.


 * Bring Vow of Silence (let the Doppelganger use it but don't use it yourself), Heart of Holy Flame, Zealous Renewal or Balthazar's Rage, Grenth's Fingers, Dust Cloak, Mystic Twister, and Mystic Sandstorm. Put all of your attribute points into Mysticism, Earth Prayers, and Wind Prayers; Mysticism is the most important. You'll need a staff with +15 or more energy on it and possibly extra energy from armor. The Doppelganger will start off by casting Vow of Silence, which means it cannot use any other spells, then you can use your skills in this order: Heart of Holy Flame, Zealous Renewal, Grenth's Fingers, Dust Cloak, Mystic Twister, Mystic Sandstorm. All of these skills will go through the Doppelganger's Vow of Silence since they are not targeted. If Mystic Sandstorm doesn't finish it off, the burning from Heart of Holy Flame will. This method takes around 6-10 seconds to kill the Doppelganger, depending on your runes and energy.


 * If you are a Dervish/Ranger, bring Avatar of Lyssa and as many slow-casting, useless spirits as possible. The Doppelganger will cast the spirits while you attack with Avatar of Lyssa on.


 * A few high damage attacks (such as Reaper's Sweep, Irresistible Sweep, Pious Assault or Victorious Sweep) should be enough to quickly overpower the Doppelganger before it can do the same to you. As also valid for most other tips in this guide, bringing slow casting and useless enchantments or spirits from your secondary profession is a good idea, since the Doppelganger will use these first, during which time you can kill it at your leisure without it attacking you at all.


 * Bring the two 3/4 second Scythe Mastery attacks, Mystic Sweep and Eremite's Attack, along with the usual slow-casting skills. Exclusively spamming those two attacks in sequence will drop the Doppelganger fairly quickly.


 * A very simple way to kill the Doppelganger in about 3 seconds is to equip Frenzy on the far left of the skill bar and Vow of Strength on the far right. Skill points should be split between Scythe Mastery and Earth Prayers.  At the start of battle simply cast Vow of Strength, attack the enemy, then cast Frenzy.  The Doppelganger casts Frenzy first, which means he takes an enormous amount of damage very quickly.

Hard Mode Notes


 * For Hard Mode, the tactics described above are usually not sufficient, as the Doppelgänger simply outdamages you. A good way of defeating him is to use Ebon Dust Aura with an Earth weapon (such as Bubahl's Grasp) and any spammable attack skill in order to constantly blind the Doppelganger. The Doppelganger will not be able to take advantage of Ebon Dust Aura. On the damage side, you can either use mesmer's empathy or more attack skills (Victorious Sweep, Chilling Victory) with appropriate attribute distribution). Of course you can throw in skills that increase your attack speed.

Another good method of defeating the Doppleganger on hard mode, is to fill your bar with some long casting title track skills. As these progress in strength according with your personal titles, they will not be strong at all, therefore allowing you some extra time to deal damage or make preparations, while the Doppleganger initiates these weaker skills.