Guide to defeating doppelganger

The following tactics have been suggested to beat the Doppelganger:

General Tips:

 * In general, the Doppelganger will equip 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. It will last into the match.

Elementalist Tips:

 * Bring lots of skills with very long cast times and only one or two "spammable" skills. The Doppelganger will usually focus on the longer 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.
 * Do not equip with skills that could disrupt your spell casting (like Gale or Earthquake).
 * Bring a lot of skills that cause Exhaustion and one or two that do not. Don't use any skills that cause Exhaustion, but let the AI do so. It will run out of energy before you do.

Mesmer Tips:

 * Bring Backfire and Empathy. The AI is not smart enough to refrain from casting / attacking while hexed with those skills and will quickly kill itself. However, be wary of them being cast on you, as they can hurt you just as much as the monster if you're not cautious.
 * Power Spike and Diversion are very useful if coupled with an few long casting Elementalist profession skills. The Doppelganger will tend to use long casting skills early, so the two skills can be used effectively. Be careful not to use the Elementalist skills yourself, because the computer AI is deadly in detecting such spells and interrupting them.
 * Illusion of Haste is often a good idea. The Doppelganger will waste time casting it (and give itself an Enchantment you can Shatter), but since neither of you is likely to be chasing the other, it will do it 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 it yourself).
 * Be careful if you bring any skills that damage the opponent's energy, such as Energy Burn or Energy Tap. Although the exact amount of energy that the Doppelganger has is unknown, it's almost certain to have more than you.
 * Ether Lord can be extremely effective or completely worthless, depending upon luck. If the Doppelganger casts it early on, it will have a distinct energy disadvantage. You may wish to combine it with Mind Wrack in hope of the AI draining its own energy with Ether Lord, thereby triggering Mind Wrack.

Monk Tips:

 * 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.
 * Bring smiting spells. You can reportedly cast them faster than the AI will. You can easily defeat the Mirror Self by only using Zealot's Fire and Reversal of Fortune.
 * Bring useless maintenance spells. If you bring at least 4 then the AI will cast them all on itself and have no energy regeneration (or even negative if you have more than 4). Divine Boon will cost them extra energy when they heal; when they run out of enery they will stop healing and have no benefit. Life Attunement will make them deal less damage and since they cannot heal due to lack of energy they get no benefit. Blessed Aura is good too, since they cannot cast enchantments anyway. 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.

Necromancer Tips:

 * Equip hexes such as Malign Intervention that take time and energy to cast but won't accomplish anything useful in the one-on-one fight. The AI will waste time and energy casting them on you.
 * Bring spells that require a health sacrifice: the less useful in the one-on-one fight, the better. The AI will waste its own health casting these.
 * If you bring lots of health sacrifice skills, bring Awaken the Blood so the Doppelganger damages itself even more through sacrifice.
 * Don't bother with 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, and so on) will do you no good.
 * For a necro warrior, equip a sword and shield. The Doppelganger will have an axe and bow.  You will have a distinct armor advantage.
 * Even though it carries a healing component, Order of the Vampire is one of the most effective skills to bring, because it carries a relatively high health sacrifice and only lasts 5 seconds; the Doppelganger will usually recast it very frequently, damaging itself in the process far more than it damages you. Dark Fury also works in much the same manner, but is exceedingly difficult to obtain by the Doppelganger battle.
 * If you find that the Doppelganger attacks more than it casts, bring Enfeeble and Shadow of Fear. If you find that it casts more than it attacks, bring Desecrate Enchantments and a lot of worthless enchantments.
 * Take all points out of Soul Reaping before you go in to battle. They won't do you any good.
 * If you have a reliable way to regain energy (like many Inspiration Magic skills, or certain Elementalist skills), bring Malaise. The health degeneration will hurt the Doppelganger more than the Energy degeneration will hurt you.
 * If you are a Monk secondary (or a Monk primary and Necro secondary), Rend Enchantments is very risky but can be very effective. Give yourself a lot of Enchantments that will not benefit the Doppelganger when it uses them on itself, then allow yourself to be hit with Rend Enchantments. The resulting "recoil" from Rend Enchantments will do a very large amount of damage to the enemy. It is important, though, to make sure that you bring enchantments that will not benefit the Doppelganger when it casts them on itself, as you do not want to bring any other methods of Enchantment removal (lest the Doppelganger cast them instead of Rend Enchantments).
 * 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.
 * 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.
 * All in all, the most important thing to remember as a Necromancer is that the Doppelganger will hurt itself more than you will be able to. Human necromancers have a difficult enough time balancing the proper blend of sacrifice and benefit; the AI will not realize that it is killing itself and will merrily spam the sacrificial skills as fast as it can for the entirety of its short life. Certain builds can induce the Doppelganger to kill itself without the player doing a single thing.
 * Bring Price of Failure and unequip your weapon if you intend on mostly casting.

Ranger Tips:

 * Bring 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 them while you apply the beatdown.
 * Bring traps. Cast them at the beginning of the fight while the Doppelganger is busy casting them himself, then run back. The Doppelganger will follow you in a straight line and is not smart enough to avoid your traps.
 * Longbows and slow-firing bows such as Horn Bows are generally a bad idea. Bring the quickest firing bow that you own (Short Bow, Half Moon) so you can get off as many shots as possible while your double is casting Nature Rituals/traps/etc. By the time they get around to attacking you, they're nearly dead already.

Warrior Tips:

 * Equip Frenzy but don't use it. The AI will use it, putting itself at a significant disadvantage in the one-on-one fight.
 * Bringing Hammer Mastery or Swordsmanship skills exclusively does not work. Despite wielding an axe, the Doppelganger will be able to use those skill nonetheless.
 * Activate skills in the most effective order for a fast take down. You know that Sever Artery works best before Gash but your opponent doesn't.  Similarly, you know not to use Final Thrust until your opponent is below 50% health, but your Doppelganger doesn't and will waste a lot of adrenaline by using it prematurely.
 * 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.
 * Bringing Healing Signet could be useful if you have a fast attack rate or high damage hits. Once the Doppelganger is hurt, it will cast it upon itself. But with the -40 armour that the signet delivers during the duration of the spell, you can deliver more damage than it can heal, entering a repetitive process until he dies. This is a good time to finally use Frenzy.
 * If your secondary class has them, bring high energy spells that don't really do anything. For instance, if you are a mesmer secondary, bring imagined burden.  It uses 15 energy, and slows 50%.  That won't matter since you aren't running anywhere, but it will use 15 energy from the Doppelganger.  This may be appropriate for other classes as well, but has only been (very successfully) tested on Warriors.