Servant Of Isiloon
Well here is a bug where you can kill Servant of Isiloons without taking any damage.
You have to go to Hell Abyss bottom level and You have to stand on the coordinates 420,339, if you do so you'll take no damage and you'll be able to kill them easily. Although you'll need to be a ranged attacker (mage,archer). They drop High class items(Shells,Raptors,shards etc.) although i don't know how often they drop them.