This has been a persistent bug since Craglorn patch with the Player bought Mercs/mages. Occasionally they will bug out and become invulnerable. Even other NPC's cant kill them. While love using the mercs, (as intended). I would rather have them completly disabled until this bug is fixed.
You can see there are about a half-dozen bugged NPCs. This is really just game breaking ZOS, why not just remove them until u can fix it???
This was one NA Server, Auriel's Bow.
http://youtu.be/MxPeSo9fD4Q