Well wrong forum i think
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but......
I have fought the doppleganger for 4 characters
2 w/mo's 3 seconds each
1 necro/mo 14 seconds
1 monk/mes 38 seconds (could probly get tiem down if i tweeked it)
Ill try to give you my basic strategy and you may have to go get some more skills
First thing first there are some things about the dopple you should know
He is an exact duplicate of you and yoru skill bar
Second he will try to cast any and all buffs on homself you have in the skill bar (this can be used either to get him to cast nedless long casting buffs leavign you a large window were his dmg output is 0 and you hammer on him
or put buffs that are counterproductive for this situation example there is a skill in the mnk tree i used that makes them deal less dmg but get healed for more when healed
3rd if you bring a healing skill he will heal i never take any healing skill with me to fight it just slows things down
4th and final any buffs you need to cast cast them right before you step on the third stone and you will pop up ready to fight fully buffed after the cutscene
basically i pick 1 hi dmg skill this is what im gonna use to kill the dopple only 1
on my warriors i took a stance as well for for higher speed output and so the dopple takes more dmg (frenzy)
Now for you specificaly i woudl say play hard to your necro side i would find one dmg skill the one that causes degen for him and regen for you (aslong as you keep this on him at all times it should cancel each other out but im assumeing there will be times he doesnt reaplie to you before it expires giving a small edge
next i would load alot of sac skills into my bar such as the one that heals minions (youll have to forgive me i havent played my necro in weeks and spell names are gone form my mind)
its early here lol if you have any other questions post here i will do my best to answer