Can't Kill Boss

I am currently doing the quest "Beneath the Depths", I am on the part where you have to kill the Treasure Guardian, either he uses his special move and fall in lava or get stuck in wall, or just re-spawn when he's about to die, I have tried to kill him many times but he won't die, any help?

 

Try to lure him into a place where he won't get stuck. For example, the little hallway. Just try to avoid his high damage. You can always lay off the quest until later, of course.