When I beat Chester Droors (a while ago) I asked my friend to help me, and she brought one of her friends. It took us an awfully long time, but we beat him. Then, when my brother needed help, I helped him and it took hardly any time at all. All I can say is, bring a friend or two! Don't try to solo it, unless you want the challenge. Oh, and load up on death spells. I recall that he and his minions are all life.
Load up on charms. That's the key...hit him with at least two... one to use and one to stay put, that stops him from healing himself. Also load up on prisms and use them. And use your secondary school spells as well. It can be done, it's just not easy. :)