Several people have had problems with certain NPCs not taking damage, in particular that shaman has come up a couple times. In one case just leaving the fjords and returning later he started taking damage normally, while another case someone jumped past him to continue in the fjords, and killed him on the way back up the trail. Someone else had the same problem with one of the boars in Broken Valley.
In general, opponents do not take damage if they are at the very edge of their range, presumably so that you can not get them to chase you, and easily take them out with a bow or magic when they stop.
There is a fight after you enter the summoning chamber, as well, which could give you a few experience points before you confront Laiken in the throne room. I assume you've finished off pretty much everything in Broken Valley and Sentinel Island? There is one quest in each area that you can do after getting the battle tower, but most areas and NPCs will be unavailable then.
