I beat her for the first time last night.
Aside from an initial StoneSkin/Blade Barrier and a couple of StoneSkin contingencies, I don't remember her having much protection. I assume she's immune to at least +2 or better weapons and her MR seemed pretty robust.
Rikter the Paladin with Crusader +5/Runehammer +5, Sarevok with Crom Faeyr and Valygar with Flail of Ages/Axe of Unyielding turned her to oatmeal fairly quickly, and in the last fight I called in a Deva, a Planetar, a Cacofiend and an Ettercap for crowd control. Aerie and Imoen contributed Hastes and blew away summoned critters, and Aerie used cures and healing spells so the tanks could concentrate on Greater Whirlwind-ing.
What are you hitting her with that's failing to hurt her?