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.
 * Equip lots of slow-casting self-buffs: the less useful in the one-on-one fight, the better. The AI 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 cutscene. They will last into the match. Cutscenes no longer run in real time, so any "duration" enchantments will not expire or run down their time during the cutscene.
 * Dodging arrows: Stand as far away from the doppelganger as you can be and still cast spells on it, and tap Q and E (Or strafe left and right, if you have them rebound) as fast as you can. Tap them both, many people hear that and try to only tap one. This twitchy movement will cause the AI to shoot wide and miss most of his shots (You can do it slower, but doing it really fast takes less practice)

Elementalist
For E/R: Bring 5 nature rituals, Meteor Shower, Searing Heat and Fire Storm. It will start casting the rituals and in then you have enough time to nuke him. But all Attribute points on Fire Magic and Energy Storage.
 * 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. For example, Fireball and Fire Storm are good choices if you take Meteor Shower - hammer at it while it casts Meteor Shower, then dodge out of the way when the skill completes. By the time it has finished you will have a huge health advantage.
 * Do not equip skills that could disrupt your spell casting (like Gale or Earthquake).
 * Bring Iron Mist, and no lightning damage.
 * Mark of Rodgort and a fire wand/staff - the Doppelganger doesn't have a fire weapon so you will have the advantage.
 * An E/Me could take their chosen element and Physical Resistance, with 0 Inspiration Magic, and leave the doppelganger to use it (similar to taking Frenzy on W/X and X/W). The extra damage caused should give you an advange over your doppleganger.
 * An E/Me bringing Blinding Flash and Empathy, causing Doppelganger to miss with his attacks while dealing damage to himself at the same time.
 * Bring flare only and spam doppleganger. Use highest on fire attribute and rest on energy Storage. Slowly but sure.

Mesmer

 * The AI generally uses skills left to right so put lots of useless, spammable spells on left and Backfire on right. If you cast Backfire in the first few seconds the doppelganger usually dies in about 6 seconds. Usually, only 2 skills in needed,  Backfire and Westral's Worry with max domination and rest to fast casting.  Cast Backfire immediately after you get in, and start wanding.
 * 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 class 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 doppleganger 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 IoW to heal it.
 * 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 chasing the other, it will do little good.
 * Don't bring a lot of 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 (let the AI use this skill, ignore using it yourself).

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.


 * Consider Retribution. The AI does not cast this enchantment on itself, and is a good way to deal extra damage without any casting costs.


 * Bring useless maintained enchantments. If you bring at least four, then the AI will cast them all on itself and have no energy regeneration (or even degeneration if you have more than four). Divine Boon will cost them extra energy when they heal; when they run out of energy they will stop healing and have no benefit. Life Attunement will make them deal less damage; since they cannot heal due to lack of energy they get no benefit. Blessed Aura is good too, since they cannot cast enchantments due to the afore mentioned lack of energy. Holy Veil is good if you don't plan to use hexes.


 * If you are a Necromancer/Monk or 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.


 * If you are a Monk/Necromancer or Necromancer/Monk, bring useless enchantments, such as the ones mentioned above, and Rend Enchantments. Cast the enchantments on yourself and the doppleganger will lose over 200 health when he casts this spell. Also, bring other skills that require him to sacrifice health, such as Defile Flesh, Dark Pact, etc.


 * The doppleganger's AI is not aware of the Divine Favor healing bonus. You can use cheap, seemingly useless spells (e.g. Remove Hex) to heal yourself, while the doppleganger will never do so.


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

Necromancer

 * Bring spells that require a health sacrifice. Don't use these skills yourself, but rather let the doppleganger use them. By including Awaken the Blood and/or Cultist's Fervor as the first skills in your skill bar, the doppleganger will use them first, thereby increasing the health sacrifice. An N/Mo may wish to bring Scourge Sacrifice in order to further increase the health sacrifice.


 * On top of skills that sacrifice health, skills that inflict conditions upon yourself are great opportunities. You are smart enough to stay away from Signet of Agony and Chilblains (not to say that those skills are not useful, merely that they do not help in this particular situation), but the AI is not, thereby cursing itself with Poison and Bleeding while you only take a small amount of damage.


 * Enfeeble and Insidious Parasite is a possible combo. Keep them on the doppleganger and let it attack you in melee.


 * If you are a Necromancer/Monk or Monk/Necromancer, bring multiple monk enchantments that are useless for the battle along with Rend Enchantments. Cast the enchantments before entering the battle and let the doppleganger rend them thus damaging itself.


 * Don't bother with vampiric skills or hexes such as Life Siphon or Life Transfer. The AI 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 Wells, any Minions, Soul Feast, Consume Corpse, Necrotic Traversal, etc.) will do you no good.


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

Ranger

 * Set Beast Mastery to the max you can get, Bring a pet with you (level 20 if possible), and take only pet-attacks. The doppelganger won't have a pet, gicing 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 AI will waste time casting the rituals while you apply the attack.


 * Longbows and slow-firing bows such as Hornbows are generally a bad idea. Bring the quickest firing bow that you own (Shortbow or Half Moon) so you can get off as many shots as possible while your double is casting Nature Rituals or traps. By the time they get around to attacking you, they're nearly dead already.


 * Equip a lot of effective bow attacks, once the battle begins, charge right in next to it and attack, because it will disable him/her using bow attacks, since they will equip their axe.

Warrior

 * Equip Frenzy but don't use it. The AI 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 (ie Power Attack, Seeking Blade/Irresistible Blow/Swift Chop, Griffon's Sweep, etc.) and the Doppelganger should be down in 15 seconds or less.


 * Bringing Hammer Mastery or Swordsmanship skills exclusively does not work. Despite wielding an axe, the Doppelganger will still be able to use these skills.


 * Equip your attack combo from right to left e.g. Final Thrust, Gash, Sever Artery. The Doppleganger doesn't understand the order certain skills should be used in, and just uses them from left to right on the bar and when they are charged.


 * Any skills with a "Lose All Adrenaline" component, such as Wild Blow, can be a great benefit. Ignore them yourself, but let the AI waste its own adrenaline using them on you.

Ritualist

 * Equip Soothing, Shelter, Shadowsong, Pain, Bloodsong and Dissonance and bring your Communing attribute as high as possible. As soon as the fight begins start summoning spirits, starting with Dissonance (to keep him from summoning a spirit army himself) and going backwards in the list. You don't need Soothing and Shelter, they are only equipped to keep the Doppelganger busy and reduce the chance, that he also starts with Dissonance. Typically he will be dead within 25 seconds.
 * Equip only Vengeful Weapon, and have max restoration magic. Repeatedly spam on yourself for an easy win.

Assassin

 * Bring a pair of vampiric daggers and no skill at all, max critical strikes and dagger mastery, you're done.


 * With all points in daggers and critical strikes, you can outdamage the doppleganger if you have the +15 while attacking armor.


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


 * As a general rule, if you can start a dagger chain that you can use but the doppleganger will not, it is quite easy to outdamage it. Falling Spider and Horns of the Ox make a great combination that can be alternated to keep an ongoing chain of damage and degen on the doppleganger. By utilizing methods to knock down the doppleganger (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 A/R and bring a pet. All you need is pet attack to cause a condition, follow with Iron Palm, and now you have the knock down to start the chain.


 * 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 degen. Dodge arrows while it is poisoned to reduce damage. 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 as your only skill with the Shadow Arts attribute at 0 while having good Dagger Mastery and Critical Strikes and wait until it ends for him. Do not use Shadow Form at all.