I assumed damage from retaliation is always physical.
But whatever, you still can kill fireslugs or any enemy by applying the Decaying Touch on it. It saves against magic armor, so you will have to break it first, which might not be easy, but after that they will damage themselves, right?
Actually no, decaying touch only affect healing spells or potions, no all healing.
I am not 100% sure but really think Healing from higher than 100% resist in an element still heals you with decaying touch.
Healing from necromancy spec or skill (mosquito swarm, pool of blood) still heals you with decaying touch so you literally have nothing to stop it.
