Live behind her. Add some lightning or magic to your attacks. Two-hand your weapon. A lot of her attacks are frontal and have no extra area effect. If she spews lava, back up somewhere and lure her away from it. Get behind her again and keep circling as you hack away. If you have any Humanity, reverse your Hollowing and summon some phantoms. Get two real, live actual people or at least Mildred the Maneater.