I don't know if you've already managed to beat him, but I also had the same problem, so I thought and instead of using the potions during the battle, I used it before on the Earth and Air element that are the most affected, in addition he is weak against lightning .
Spam Lightning and stock up on potions, every time you have a break from fighting use your potions. You need to have atleast 8.0/7.0 in everything before fighting the boss. During the boss fight spam lighting and you win.
