Board Thread:Skyrim/@comment-1188343-20150720120617/@comment-6881629-20150721120007

If he is stuck at about level twenty he should only have about one hundred and fifty magicka, with just destruction skill and no perks thats probably only enough to use the bolt spells about three times (maybe they always have the more advanced spells but not the magicka to actually use them until later levels).

I would guess because the Wabbajack removes a npc and replaces it with something else when they reappear it might be a new version of that npc so their level is updated, no idea about thralls but it doesn't look like the npc is replaced.

Rulindil is apparently skilled in heavy armor and one handed that might help his survivability.