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?
I always like to brag with how little meta-knowledge I had to solve quests, but with the Last Light I could draw on the experience of maybe a dozen ways of how not to do it after just entering the Shadow-cursed Lands. My in-game explanation for waiting with visiting Isobel until almost the end of Act II, was that we were technically on team Shar and doing it could upset Shadie. Then, we returned fully buffed and knowing many secrets.
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