Isn't that always the case if you leave an enemies attack range? It makes sense that anyone trying to pass a ball of fire gets burned. Hmm, I'd say this is the biggest challenge so far. We can't talk sense to the guy, we can't even attack him right now and even if, unarmed strikes and grappling means fire damage. Does the stone trick that worked with the acid slime work here too?
For now we need to get away from that fireball. He could chase us into the bulette nest. If we split up the fireball can only go after one and the others can go back and attack the skull. Not sure if this is feasible, I guess it has more kaboom magic.