Don't worry about his resist. Just put out a prism and wail on him. You will want to take some friends. The battle gets progressively easier the more help you have. Besides you can't even get in the final door without at least two others with you anyway. Not to say that fighting him alone isn't possible, just much more difficult.
Try to use spells other then deth spells. Beacuse I am deth and I deafed Malistare on thefirst try! If you want friends I would want a myth wizard beacuse they can summon creathers to help in a battle! So give it a try. :D [