There are 2 ways:
1) Just brute force it. Keep spamming celestian form, until he is out of Mana. When your HP drops to 0 in celestial form you return to normal form with full HP, so you can just keep using it over and over whenever you get low in normal form.
2) If you have the succubus skills (obtained from succubus clothe set) you can equip the related skills and drain his mana to 0 at the start of the fight, then he won't be healing. Its the faster method.
