I had the exact same problem.
Playing on hard and found the fight difficult but finally got it done and now it is still pending in my quest log. I had explored all options, so talked to Raedric then went to Kolsc in the abandoned building then went back and killed Raedric but the quest log still has that im deciding whether to kill Kolsc.
From what I read he should've triggered when I left the throne room with Raedric dead and I cant find him anywhere, in the abandoned building only Gaellic (sp?) is left.
Really disappointed because I have no interest in redoing that fight, and if I cant find a solution to this bug it's probably enough to make me quit this buggy game.
