It's possible on normal difficulty, seeing as I did it just an hour ago. Well, if you leave your team down the stairs and send only one person to talk to Raedric. I tried to keep everyone dazed or blinded as long as I could, I used up most of my scrolls (Rolling Flames ftw), but really it was just a matter of luck because I haven't quite got the hang of aggro yet. When I didn't manage to grab enemies with Eder, I promptly got slaughtered.
Oh yes, totally possible. Again, I just did it. 4 man party - couldn't do it 3 man so I got a level 1 priest from the inn, though everyone else was level 3. Kept her behind Eder, with Aloth taking down the priests first from range. My Cipher PC got knocked out in the heat of battle, but her twisted body made a nice block in the doorway