I went through the doppleganger a long while back, way before anything in the game was programmed to run from damage pulse spells. He folded easily to Meteor Shower & Co., if memory serves me correctly, and netted me the bonus on roughly the third try.
Later, I took my ranger through and figured I'd just fill my bar with useless skills. Nature rituals had the longest cast times, so I figured I could pummel the doppleganger a good bit while he cast them. Sure enough, the fool stood there, dropping one spirit after another.
But my recent attempts at rolling the Hard Mode doppleganger with my E/Mo were far less successful: he was able to outnuke me, and he had been patched to use Healing Breeze, so he was bound to outheal me as well. With quite a few things already on my "to do" list, I decided to leave the matter for another day.
That is, until this thread reminded me of the way I got my ranger through Augury Rock. Being the crazy idiot that I am, I had decided to start buying regular tomes, and had coincidentally unlocked all non-elite ranger skills on my E/Mo since my last attempt. Packing 5 nature rituals onto my bar, I brought along my level 5 pet and two skills as a backup plan: Mark of TROGDOR and Mind Blast.
And he fell for it! Like a good little AI bot, the HM doppleganger diligently cast all the nature rituals, stopping between them only long enough to fire off a Mind Blast in my direction. I wasn't beating his damage output, but I was casting more frequently and he was constantly burning. Then, he decided to drop both Predatory Season and Winnowing, which made my level 5 pet eat him
Mission, bonus, 300-something XP for the kill.