If an opponent is at the limit of their range (they will not follow you more than a set distance from their starting point), they will not take damage.
A couple people have also reported specific opponents being invulnerable, but IIRC leaving the area and returning would generally fix the problem. One of these cases was with the imps at the start of the fjords (near the teleporter); they bypassed the area by jumping off the mountain, and when they worked their way back up later, didn't have any problem doing damage.
FWIW, when I started leveling in the fjords, I found a level 2 ranger head on my creature much more helpful than a level 6 warrior head, when fighting a group of level 21 imps at level 17.
If you are going to start killing bunnies, you would want to do so before getting the battle tower, as most are in Broken Valley, and will not be available later. If you kill enough of them, you can trigger the appearance of the
Killer Rabbit (save periodically), which gives a nice bit of experience.
Did you find the cave on sentinel island under Sassan's grave? Jump off either side of the peninsula to find it.
Did you use that salve heading up to the wyvern's nest? If so try returning without it, and kill the wyvern that attack you; at level 17 you shouldn't take too much damage (IIRC they are level 16), and should be able to take out the majority with a bow (a static charge or retaliation aura would help dealing with those who don't stay in range very long).
In the quarry tower you should be able to take out the first 2 black ring members (use your creature to help, or at least as a distraction). I went upstairs after that (level 16 or 17) and was able to take out the 4 rangers who come upstairs, but had to duck out a side door to avoid the 3 warriors downstairs after that.
