Another cheese strat: kill the two gunners, then lure the boss to the tent next to where they stood. Get her to walk onto the top, and quickly go underneath and stand to one side of the tent. If you stand on one of the sides that doesn't have a ramp, she will try to hit you with her combos, yet every attack will miss except for the gunshots (which can safely be blocked as they will be the only source of posture damage you will be taking.) Between her combos, you can jump straight up and slash her either once or twice (depending on the combo used). It takes a long time because the jumping slashes don't hit hard. But it's safe, and was how I killed her the first time in NG and then again in NG+. Just keep jumping and slashing her after each combo and you can then go up and deathblow her when she is low enough.