The Server Setting “NPC Damage Taken” also effects Thralls. This should be divided into two settings. All NPC Server Settings should be separate from all thrall settings.
Wanting to make the server more difficult by making NPC tougher, but it also makes Thralls much tougher, which is not needed (and detriment to gameplay).
Another way to do this is to reduce player/thrall weapon damage, but this creates other balance problems with weapons and settings.