He's just standing there taking damage. I remember other times I fought him (and lost) he cast epidemic of fire and used scrolls to buff his minions. I know that if I let him reach round 3 without knocking him down I auto-lose (playing as Lohse, perma-possession). His minions are not so tough, and if I focus fire on him I can take out his armor and knock him down just in time, preventing the possession, but even after he gets back up he's still just standing there instead of fighting back. This did enable me to win, but it felt like less of a challenge that way.
