I found a tip on the Steam forums. I used it a LOT on the DLC. Stand at the door, turn around, cast about 50-100 whirlwinds, turn back to the door and open it. They will sail into him when the indicator goes red. It will bring him down enough you should be able to survive.
It only gets worse from here, even after you get your stuff back. The DLC is very hard, but I managed to get through somehow (a lot of potions & luck/reloads).