That battle involving Markthum and the others is bugged. Markthum does over 1600 damage per hit, which really doesn't make sense. He's so overpowered he makes Behrlinn and Ygerna look like a couple of imps.
Thanks. When I encountered Markthum and the others I was above level 30, and he's level 22. I almost couldn't believe it when he instantly one-shotted me but I checked my combat log and sure enough, 1600 + damage every time,and one of the others was also doing almost as much damage.
Oh, I didn't realise this was a known "feature". I had a real sulk when I took them on at level 25 or 26 and repeatedly had my bottom handed to me, eventually having to settle for running into one of the towers and sniping.