It's fairly easy to kill the Wizard that Edwin sends you after, if that is who you are talking about (if you mean the lich - don't even try it at level 10 because you *will* die). Send one character upstairs to meet him (make sure this is a fairly hard character who is resistant to magic if possible and that you have another character with a spell of restoration handy!) and then let the wizard cast all his spells on this character. Once he's worn out all the spells or the character is dead, send the rest of your troops upstairs and then use general tactics for dealing with wizards - i.e. attack with all your fighers and summon monsters/cast counter-spells with your wizards/sorcerors.