Go in with full ultimate, atronauch. Pet can get her occupied pretty well.
I dunno if this is the way to go, or if I exploited the thing. Tharn never leaves the platform, so I kept shooting him with my Silver Bolt, and killing the skeletons who came after me in melee, making sure there never was two at the same time.
This took me a good while since the bolts chew stamina but I don't have any other ranged ability. One time I got a bit to far and Tharn reset, but in the end it was doable. During all this Lyris stood in front of me and just watched with her weapon on the back.
This was not a fun fight, nor did it feel right. I also feel sorry for the poor sod entering this with no ranged ability at all.