For Diablos just Draw and Stock Demi spells, then cast the ones in your stockpile. Repeatedly doing so will bring Diablos' HP down low enough to be finished off with physical attacks. Also, whenever Diablos is hit by a Stocked Demi he will cast Curaga on the characters, so you'll always be at full HP. Blind can help reduce damage from his physical attacks if you're having trouble drawing Demi spells from him.