Be a high level. Buff yourself, but dont overcommit turns to buffing, since he can remove them. Make sure you have the Virgin’s Cross buff up at all times, since many of his moves are Darkness based, including his ultimate. Make sure that your equipment prioritizes attack, since the boss will always deal about 7% minimum amount of damage, despite your defense. Make sure all your equipment is equipped with high level gems.
I have a walkthrough available for download if you want a bit more help, but that’s all I can think of off hand. Good luck! :)