Things You'll Need
Instructions
Cast Mana Shield on yourself. This spell causes damage you take to hit your mana pool rather than your health.
Move away from the other monsters around where Diablo appears. Diablo follows you, so you can wait here until he approaches.
Cast Fire Wall on Diablo. This spell creates a wall of flames that damages anything standing inside it. Keep recasting the spell after the effect ends.
Use mana potions to refill your mana, which is going to deplete as Diablo casts spells on you and you use Fire Wall. You just need to keep your mana from depleting until the Fire Wall kills Diablo.