Yeah as far as this fight is concerned... The fast and furious way of doing it is to just kill the member who is possessed, then resurrect them. IIRC, the demon takes damage every time it is forced out like that. So just have a bunch of resurrection scrolls on everyone and make sure you have some good burst damage to land on the demon in one or two actions. Again, IIRC, the demon will possess another guy at the same time the second attack hits him. First time I killed this guy I only had to kill two guys of my party. If you have only one guy left and he's possessed... I don't think there's any way out of that, except by cheating, maybe.
Otherwise, to forced the demon out without killing the possessed member, you'd have to keep damaging and healing them at the same time. After a certain amount of damage is dealt, which is (I guess) based on the base HP of the possessed character, the next attack that brings the character to "near death" will forced the demon out. Soooo my advice is, prepare enough healing spells. Keep cc'ing the possessed char (without dealing too much damage) until your healing spells are back up. At least you have only one target, so it's not that hard to chain cc them. Teleport the possessed guy down into the pit whenever you can. Unless he has a teleport-type spell, he can't get out of there. So your team can stay around the ledge and bring the pain down.
Damaging the character while keeping them alive at the same time might be a pain cause there's the dwarf guy who will also throw his stuff at the character (at certain points I really, really wanted to kill this guy too!). So another good idea is to save the game like at every turn, in separate slots. The second time I killed this guy I managed to keep everyone alive, but the fight lasted for quite a while, and I had like 10 save slots just for this fight.
Anyway, I'd say that the key is to chain/cycle your cc's and healings in a reasonable way. Having good burst damage helps keep the fight short, but is less important.
There might be some "trick" ways to do this, too. I once managed to get the whole team out of combat, with only the Bone Widow fighting the demon. The Bone Widow can't be possessed, and is strong as hell, so it actually managed to get him to near death, then my party just went in and cleaned up. This was before the Bone Widow was nerfed, of course.
You might also want to try Peace of Mind, see if it works. The demon will possess the one that last attacks it.
Last edited by Try2Handing; 24/10/17 08:00 AM.