If it's Josephine, as Raze suggested (and who I was getting confused with) I think I used a combination of a sword with a suitable stun enchantment and lots of invisibility potions so I could sneak up on her and quickly disappear again once she had recovered. And lots of health and mana potions, of course. It was a bit of a grind but I got there in the end. She is a pain, though.
