Hot Head can only be damaged by fire from the Magic Rod. Stay near the door and spam the attack as he jumps out in front of you. Hot Head's movement is RNG, so you will also need to be a little lucky. It is possible to stunlock the boss. If you shoot fire to the opposite side of the room and the boss moves into it, the boss will be stuck in place as he takes damage. Though unnecessary, it is recommended to have the Roc's Feather equipped in order to jump up if Hot Head returns to the lava near you. A single splash of lava does a total of four hearts of damage, more than any other boss in the game.