Sometimes the Labyrinth Horror shows up when fighting him, and there is always some ass that opens the other door for the veteran skeleton. All the guardians are suppose to use wall of reflection on his candy corn bursts, and you're suppose to melee him, you're less likely to get hit oddly enough.
That happened a few times. during these zergs I realized my minions were nearly pointless. I needed more health than the one could provide. With Well of blood and power and my shrapnel grenade. Every skill and attack except 1 would hit groups of enemies or allies.