Yeah, it's bugged. You can turn it in with your quest log only crediting you for 1/2 kills. It's a display bug, but it's actually done. Go to the class trainer and finish the quest.
What's actually happening (or at least happened to me) is that you already killed those 2 mobs that you needed to kill, but when you loot the boss' head, the key, or open the chest, that action subtracts 1 from the kill count in your quest log for some reason. I actually saw with my very eyes as the kill counter went from 2/2 to 1/2. It's probably a badly copypasted quest script. But the game still considers that part (the kill 2/2 mobs part) already done.
So there is a desynchronization between the quest logic and the display. Because you already killed the 2/2 mobs, you cannot get credit for it for killing them again, and so in your quest log the kill counter cannot go up from 1/2 to 2/2, even if you kill those mobs again, because the logic considers that part complete.