In D2:DE several people reported specific opponents being invulnerable, but IIRC leaving the area and returning would generally fix the problem. A couple these cases were with goblins the start of the fjords (a goblin chief near the teleporter, and another at the same spot as you); one person left and returned later, while another just skipped that section of the path by jumping off the mountain, and when they worked their way back up later, didn't have any problem doing damage.
Have you tried magic or melee attacks, or summoned your creature to see if it could do damage?
Since you are past that section of the path, try returning later.
If you are playing the PC version, a save to support@larian.com might help them figure out what the problem is.
The number of imps along one of the paths was reduced and the flying fortresses streamlined a bit in DKS compared to D2:ED, so the extra experience points may just be balancing that out.
Welcome to the forum.
