Using a sleep spell seems to be a strategy. though it is very time consuming even though she is weak to it.
There is a weapon you can get from the church called "Demon Layer" which is a whip that is specialized in holy attacks. She is also very weak to it. The only problem though is that if you hit her 3 times with it she defeats you instantly.
My strategy with the fight was using a power up along with a crit multiplier at the same turn and Hitting her with it. If the crit appears you take out so much of her health. The fight is pretty much over if you know what you are doing.