In case it wasn't mentioned already you can sneak up on her from behind(take the first path down instead of the one with the flower trap) and cast silence where she is standing(her line of sight gives it away). Then she won't summon the masked slaves and can't cast anything as long as she's in the silence bubble. If you don't want to kill her, make sure she gets out of the bubble when she's low on health, or she can't initiate dialogue.
