Which puts more credence to the idea that she was a near epic level encounter even though the adventurers were not in her league. Watch the fight scene. It wasnt about killing her in a grand battle, it was about disrupting her plans in such a way that it backfired. Aside from chucking the flesh to stone device at her, which failed to work as she made her checks after the initial effect kicked in, they didnt really fight her at all. Heck even the dmpc sorceress that helped willow was defeated in a fight with the bbeg.