Hmm, I'm going to go out on a limb and guess this was:

I just persuaded my way through that one. Didn't even try the violent approach…

For the Mordus fight I just prepositioned everyone by sneaking up onto a ledge behind one of the flying possessed on the northend of the cavern. Initiated the conversation from there. Mordus teleported up to the group there and got swifting creamed by the group. Once you kill him his minions insta-die. Mordus didn't even get the chance to transform.

The castle before the cave was pretty tough for me.

Did you go through the front door or from the mountain side? I just hung out up there and drew them in one by one with a tank.

Mountain side, but I didn't see any way to draw them in one by one. Now that I'm more experienced, I'm pretty certain I could have used teleport to at least pull one or two out of the gang but I didn't understand the agro mechanics back then.

I kept the tank on the top platform who aggroed the closet with a ranged spell and then pulled back into the southeastern corner. All my other party members stayed on top of mountain near the climbable cliff. Also prepositioned a few poison barrels around the platform for some glorious poison explosions. Fun times! :)