-( Story related, cannot be missed )-
Ah, the elusive mistress of night turns out to be our departed sister, turned unknowingly by our own fangs. She bears malice towards you and seeks to take it out on you in spades. The fight will be no different from any other in regards to dodging being your savior; that being said, Mary herself is a potent enemy, born of your blood much like you were born of your Maker's.
She will lead up with a simple one-two or one-two-three combo with her cross, which can build up if she does it enough times. Wait for her by strafing to the sides, dodging as you can, to evade her attacks and go in for only two to three hits at a time. With enough damage dealt, she may do one of two attacks after a fourth to a fifth of her health has been depleted:
1) She might do a bestial roar (akin to Fergal's, but much more potent and larger) if you're near her, so keep your distance if she stops in one place and shakes.
2) She might vomit onto the ground and create blister patches. These take a few seconds to form, then a bit longer to explode, causing blood damage in a large field and lingering for a bit afterwards.
Either of these can be life-shattering, so be aware of your surroundings if you can and try to use her and your dodging ability to destroy the tombstones, making it easier to keep your distance. One major note: the priest is still alive in the beginning of the fight. If you do too much damage to her, she will shift to his side and feast off of him, giving her a full heal, so if you don't want this fight to last longer then necessary, you might want to eat him first. Don't worry, as he is not one of the vital lives you have to spare for the "Not Even Once" trophy, so lap away at his life-giving essence.
Normally, her blister patches will form in a 3x3 grid when she starts using that mechanic, but later on, she may form some in irregular formations. Safe zones from the blisters might not so safe from her attacks. I figured I'd let you be aware of that.
After the fight, you will commit her back to the grave and this trophy will pop before that point.