Normal Mode: Brown Vial alone lets you not die in a single turn. So heal up and spam black orb.
Maso Mode: Brown Vial is nowhere near enough for you to survive a single turn.
How to Kill Heartless One
As it turns out, almost all of her attacks are magic based in nature so applying a purifying talisman and equipping a Soul Devour Necklace before going into the fight heavily reduces the damage you take.
She also isn’t immune to DoT, so tossing a Murky Vial at her for burn and hitting her with bleed if you have the option works well.
Someone does have a video up for the fight: