I've seen this posted before, and it worked really well for my Warrior/Ranger character.
Load up on junk nature rituals (particularly spirits) before facing the doppleganger. He'll waste time running through all of those, giving you a nice head start in killing him.
In my particular case, being a warrior primary, I loaded Frenzy, 5 useless nature rituals (all spirits, picked pretty much at random), and 2 sword attack skills (I've read that he uses an axe in melee, so the sword skills would be useless to him; they were for me to use

). I had him dead in about 15 seconds.