If I remember correctly I don't think that is currently possible via the plugin API, but you might be able to do it locally for your game by changing some entries in the game's definitions database. If you are interested in that way it is a bit more involved than simply installing a plugin but I might be able to walk you through it
getAttacker()Gets the attacking npc, i.e. the npc who hit the target npc. if attaking name = skeleton{ setCancelled(true) }
this would be under the NpcHitNpcEvent