These mobs appear unkillable, there are 3 of them on the way down to Hatchet's room off of the lab(door in the ballroom).
When killed they become untargetable for about 30 seconds then pop back up and you get to do it all over again.
Bringing all 3 to camp and killing at the same time then running past works fine.
The rumor that killing them X amount of times before they finally die doesn't seem to be true.
They're also NOT catagorized as undead.