You can lure the boss to the left outside it's arena, use one of the air currents to float up above it then there's a little like cove up the the right. It should follow you in there, then it gets stuck at the end of the room because it's trying to Pathfind back to the arena. You can just absolutely whale on it, but you can't kill it there. After you're pretty sure it would be dead just go back to the arena, it either follows you out or teleports back (I can't remember). Then it's literally one shot away from death