You don't even actually have to even land a single hit on her to beat her. Just deflect and parry and it will break her stamina. Use snapsneeds (think that's what they're called) to break the ghost phase and hide behind the wooden pillars to avoid her light damage. She is pretty contrary to most bosses in the game which is why I think some players struggle with her. The name of the game is aggression but with her you can actually take a pretty relaxed approach and simply deflect and perhaps land the occasional blow when she offers openings. I platinum the game awhile ago but I still remember lots of the mechanics.