Would it not take damage or just super strong but do able?
I've encountered a random bug on bosses (random as in different bosses and inconsistently) where they don't take damage. Anything like that? Did you re try it?
All my characters use heavy armour....impenetrable...
I defeated the grand sermonizer with a level 6 bosmer nb. Shouldn't be any problems
You have not been "nerfed"
You use all heavy armour + impen. There is yor problem. You simply don't have the damage you need to beat that boss.
Basicly you are running with a PVP setup, trying to do PVE and that wont work.
As @Runs already said above...
Level shouldn't really matter much since you're scaled.
You probably need to work on your rotation, skills, gear etc. ...