I think I figured it out. Companions can’t “dodge” attacks like regular players can. If there’s a giant red spot on the ground, companions act like cats chasing laser beams. Smoosh. This big boy killed me twice before this video. He came close a few more times as you can see. Pretty sure I’ll be changing my skill lineup for the next boss.
I’m 52 now (linear levels, about 4 per night), so only a few more levels to go. 2 zones methinks.