[Sean]

Spoiler alert!!!!!!!!!

Sumon up 2 or 3 creatures like Aerial servant or elementals and then cast protection from evil 10 radius. Then cast pit feind and click on the door. When Irenicus appears cast another demon spell as far away from the pit feind as possible (because you dont want your two demons fighting). Use your two best fighters to attack irenicus and have your mages cast true sight and spell protection removal spells (spell thrust etc). Try to get rid of the demons before going all out on Irenicus.
Thats how I did it any way
Good luck!