This happened to me vs Minsc, I had to literally save scum the bejesus out of it because everything I did in non-lethal killed him, even unarmed attacks. Eventually I managed to get him down by him attacking Karlach and taking a couple of points of damage from her bonespike armour, but other than that, any attacks I instigated just ruined him. Annoying bug as I really didn't want to save scum my tactician run, but I really wanted Minsc in my party.
