Well, I may be inviting lolwuts? by saying this, but the cowerstun was the death of me fighting the Nazgul at the end of the Helegrod treasury. And yes...this was with the inspired greatness uber buff. If it helps my case at all, I did defeat him the second time, after popping a hope token and using some DPs.

I think I literally got that stun like 6 times during the fight.