After many attempts, all with the same outcome, I raised my health with the command console from 150 to 1500 in order to survive long enough to see how to damage her.
She does 960 damage in one hit.
In spite of me being a Breton with Dragonskin & 40% Cameleon.
Even when I managed to get a Gods Fire spell off, it didn't even touch her, nor do magic weapons.
Apparently she is leveled (I'm on level 19) so if I go back later, she will be harder.
Has something interfered with her stats?