[Faceman]

You don't have to fight the demons, if Irenicus falls it's sufficient!
The easiest way to beat the evil wizard is to summon up some pets to keep the demons busy and flee south down to a staircase. Irenicus will be teleporting on the platform counter-clockwise. Now wait for his spell protections to wear off and conjure up some small creatures now and then to send after him, as will use up his high-level spells on them. After a good deal of time (and after you've run out of summoning spells and items) use every defensive spell/potion you have on your party and CHARGE! When you have brought down the Slayer to Badly Injured let your strongest (referring to melee) mage use any time stop spell (remember that Limited wish also allows time stop once) he has access to and whack Irenicus (Imoen with CF is very effective). This should make it easy to defeat the Exile even with a low level party.