Spent about 2 hours trying to fix this problem when I experienced it. At first I tried to fight the boss despite this glitch, but after dying about 20 times I had enough. Tried rebooting the game, then did a complete shutdown of my computer, and when all that failed I tried playing in both windowed mode and fullscreen to see if it was some sort of resolution problem. None of that made any difference. I really hope your post gets noticed because otherwise I don't think I'll be playing more of this game. After all, if the mobs spawned by this boss are broken, I imagine that other bosses will have the same problem...
