It may seem a bit cheesy... but then again.. I dont think it is. When you attack the cultists in the room above, one of them has a wand of sleep on him. When you go down to the basement, have your people start attacking the cultists, and have your mage/bard use the wand on the demon. It may take a few reloads, but it works. Believe me. Then, whilst he's napping, kill off everyone else that is around. Once that's done, just whoop the demons ass!