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.


 * For all ranged attackers, stay on higher ground. Ranged attacks from higher to lower ground do more damage.


 * 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.


 * Take your armor off before proceeding to the 3rd rock and when you arrive to fight him equip it immediately. This will give the Doppelganger 0 armor and leave you with your normal fully suited self.

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).


 * Bring Mark of Rodgort and a weapon that deals fire damage; the Doppelganger doesn't have a fire weapon, so you will have the advantage.


 * An Elementalist/Mesmer could take elemental spells and Physical Resistance, letting the Doppelganger use Physical Resistance. The extra damage caused should give you an advantage.


 * An Elementalist/Mesmer could bring Blinding Flash and Empathy, causing the Doppelganger to miss with its attacks while dealing damage to itself at the same time.


 * Bring Swirling Aura, which gives a 50% chance to block arrows. You're not using a bow, so it gives you a significant advantage.


 * Any Elementalist that wishes to use Air skills for this mission can fall back on a dual attunement Air Spiker build. This has been tested extensively in normal mode for this mission and shows that you can defeat him within 15 seconds by only using Lightning Strike and Lightning Orb. Make sure you cast Air Attunement and Elemental Attunement before you walk on the 3rd rock so that you already have these enchantments on you before you enter and don't lose time casting them.  This method does not work in Hard Mode: his increased attributes and armor make this woefully ineffective.

Hard Mode Notes


 * For Hard Mode taking Kinetic Armor, along with Armor of Earth and Mark of Rodgort combined with a fire weapon works really well. Before entering, cast both Kinetic Armor and Armor of Earth. Once inside, cast Mark of Rodgort and attack him with your fire weapon so that he will die from the burning. Keep Kinetic Armor up by spamming Mark of Rodgort when it is about to end so it will restart again.


 * Empathy builds with Blinding Flash or Mist Form are effective. The Doppelganger will use Mist Form on himself, allowing you to alternate with him to prevent any weapon damage.


 * 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.

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.

Hard Mode Notes


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

Monk

 * Consider bringing Healing Breeze. The AI currently will not use Healing Breeze for ordinary healing. However, it does 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.


 * An easy way to kill the Doppelganger is to pump as many points into Smiting Prayers and Divine Favor as possible, and bring only Scourge Healing, Orison of Healing, Mend Ailment, and Guardian. Cast Scourge Healing on the Doppelganger, then fight from a distance. Use Mend Ailment and Guardian to heal from Divine Favor bonus; the Doppelganger will not use these. It will cast Orison of Healing to heal and end up killing itself.


 * 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.


 * If you are a Monk/Necromancer, take only Dark Pact and Healing Breeze. Put all your points in Healing Prayers and Divine Favor. Let it use Dark Pact over and over, while maintaining Healing Breeze on yourself. This can beat the Doppelganger in less than 50 seconds.


 * The Doppelganger's AI is not aware of the Divine Favor healing bonus. You can use cheap, seemingly useless spells (such as Remove Hex and Mend Ailment) to heal yourself, while the Doppelganger will never do so.


 * 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.


 * 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.

This way killed the Doppelganger in about 30 seconds.
 * If you are a Monk/Mesmer you will find it effective to bring only 2 skills, Healing Breeze and Illusionary Weaponry. The healing breeze will be used for healing only by the player as the Doppelganger does not use it to heal and the Illusionary Weaponry will be used for the damage dealing. be sure to bring only these two skills or he will use other skills as well.

Hard Mode Notes


 * The Hard Mode AI will use Healing Breeze.

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.


 * A Warrior secondary and a Hammer with maximum Hammer Mastery will bring the Doppelganger down to 50% before it can get you to 50%; this is ideal for using Taste of Pain.

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.

Ranger

 * Set Beast Mastery to the max you can get, bring a pet with you (level 20 if possible), and take many pet attacks. The Doppelganger won't have a pet, giving you the advantage.


 * Bring many slow-casting Nature Rituals like Winter and Quickening Zephyr (the less useful in the one-on-one fight, the better). The Doppelganger will waste time casting the rituals while you attack. If you are R/Mo, bring some smiters like Balthazar's aura


 * Longbows, Hornbows, and Recurve Bows are generally a bad idea due to their slow refire rate. Bring the quickest firing bow that you own (Shortbow class is the fastest; see Bow) so you can get off as many shots as possible while the Doppelganger is using useless skills like Nature Rituals or traps. By the time it gets around to attacking you, it's nearly dead already.


 * Equip a lot of effective 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.

Warrior

 * Although the Doppelganger wields an axe, it can still use sword and hammer attacks.


 * Equip Frenzy but don't use it. The Doppelganger will use it, putting itself at a significant disadvantage in the one-on-one fight. Couple this with a few energy-based attacks which you can use right away (such as Power Attack, Seeking Blade, Irresistible Blow, Swift Chop, Griffon's Sweep, 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)


 * 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.


 * Bring Frenzy, Healing Signet, Distracting Blow and an attack skill in that order. The Doppelganger will use Frenzy and eventually Healing Signet, use your attack skill while he is casting Healing Signet and follow up with Distracting blow if he is still alive.


 * Equip Healing Signet and Frenzy only. Then hack away. (The Doppelganger will eventually use this "combo" and you'll get an easy job. The Doppelganger was killed in under 15 seconds using this method.


 * The Doppelganger in hard mode has a much better AI. But 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.


 * A Great strategy is to switch secondary profession to Mesmer, pump up Domination Magic to 12, and pump up strength to 12 using a Superior Strength Rune with the +1 Strength helm, ultimately giving you 16 strength. Bring Empathy, Frenzy, and Dolyak Signet. Regardless if the Doppelganger uses Dolyak Signet, it will not reduce the damage taken from Empathy. Also, keep in mind you must not attack or you will trigger Empathy on yourself (Doppelganger will re-cast Empathy as soon as it recharges). Cast: Empathy, Dolyak Signet, wait 10 seconds, recast Empathy, and before Dolyak Signet recharges the Doppelganger should be dead. Keep in mind he will use Frenzy whenever it becomes available.

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. It only took me 6 seconds to kill him. 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.

Hard Mode Notes

You can beat your Doppelganger easy as Rt/Me by using Arcane echo then Vengeful was Khanhei, keep VwK up and then keep using Vengeful Weapon. this way you can kill your Doppelganger in 1.30 minutes. you don't even need to bring any other skills

Assassin

 * Bring a pair of vampiric daggers and no skills with high Critical Strikes and Dagger Mastery. Having Nightstalker's Armor is also helpful.


 * 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 Viper's Defense, Heart of Shadow, and Seeping Wound. Never attack. Use Viper's Defense to poison it and Seeping Wound to add to the health degeneration. Use Heart of Shadow to regain health. This method is slow, so you may not get the bonus, but it will give you an easy win.


 * 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.

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.

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. Precharge the spear attacks while under Anthem of Flame and Aggressive Refrain before you enter.

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.