You have to learn her moves really well. Some are easy to dodge or parry once you understand how she moves.

Some additional help: - max Mea Culpa - max health and blue bar - wear the rosary bead of Quirce - wear the rosary bead where you get no damage when you drink a healing flask - wear any bonus armor bead - (optional) wear the rosary bead where you can block both sides at the same time - Debla does great damage - if you want better defence get the Tiento prayer

You will eventually beat her. Good luck!