I’ve said this before. If you’re thrall is getting decimated by NPCs, you might want to dump them and take a truncheon to those NPCs as replacements. Thralls have the same stats as they did when they were in the wild plus any bonuses from attributes gained.
Cimmerians (Forgotten Tribe), Votaries of Skelos, and Relic Hunters tend to be the strongest in the Exiled Lands. It makes sense they would decimate those from lesser pedigrees.