(I'm assuming this is a bug, because I can defeat anything around these parts without a problem, but not this one)

World: Nightmare
Mob: Ravenous Dead (elite)
Problem: Cannot be defeated
Symptom: Can attack and get it down to around 15% health, then a green goo-aura appears underneath it and the health shoots up to 100%. It doesn't matter how many we are, we can't dip his HP bar a single pixel.

Screenshot: http://img696.imageshack.us/img696/7109/screenshot000qz.jpg