Harvester help, please.

So I've got one achievement left to unlock, which is to beat the Harvester from Golems Of Amgarrak on Hard or Nightmare. I spent 5 hours trying last night. My warden was imported from Awakening, DW rogue capped at level 35. She did start out with Duelist, Assassin, and Shadow as her specializations but at one point I reset her specs with that tome and changed Duelist for Legion Scout. Before that, I had made Jarrik and archer with the tome. I'm not letting the companions free range,  I set their tactics and use the golem for group heals and cleansing auras only. Yet it and the two brothers get wiped in Phase 1, around the time the Harvester is down to half health. I did make it to Phase 2 twice on my own, and timed the kill switch to wipe the skeletons out at the beginning of the phase. I just can't seem to get the Harvester's health down enough in time for it to regenerate health and before I know it, more skeletons spawn. I found myself more or less running around in circles for 30 minutes or more because whatever I was (or wasn't) doing didn't take the Harvester's health down enough before I had to run around a bit to chug potions, or whatever.

From things I read, Arcane Warrior seems to be the most preferred choice for this fight, but I would really like to finish it with the warden I have. I might have a mage warden from my Awakening files (I clear them out every so often) I could use but I'm not sure what gear I carried over from Origins with her. That and I really just want to finish Golems Of Amgaarak and put this whole experience behind me. I'd only played it once before, on normal and that was before I got Awakening. I knew it would be insanely hard but I thought with the Awakening import it wouldn't be **as** bad. I have a ton of bombs, potions, and poisons on hand so I believe I have plenty of stock to make it happen with my current warden. Or so I hope!

Any tips or advice would be awesome. Thanks!