I think it has to be FF Marcus and his jolly band of winged undead... just one blown save by Isobel, or one critical hit on her and POOF you lose, sucker! Not to mention why are you all "Surprised" when it's perfectly obvious what's going to happen?
Ha, I was "surprised" in the encounter at the end of the sentient amulet quest line, despite the fact that I initiated combat by clicking the icon at the bottom left of the cutscene screen
