This short chapter of the Vampyr solution breaks down just one fight, which is, nonetheless, very emotional and shocking for Jonathan.
- Approach the priest to trigger a long scene, in which you will see... Mary! She came to life and shares Jonathan's fate - she's a vampire now.
- She has brought her mother too - sadly, no arguments can persuade her, and you will need to fight the woman. This is one of the hardest fights of the game.
- She will use a crucifix in combat. Like Jonathan, she's an ekon, a higher vampire.
- She has a few trick up her sleeve. Apart from close attacks, she spits venomous and explosive blood - you have to avoid any spot where the blood falls. The explosions deal lots of damage.
- She sometimes musters up more power and gives a wretched cry, creating an extremely powerful shockwave that can kill you with a single hit. This is a real show stopper. You can mitigate the effects with Blood Barrier and a well-timed dodge.
Once you deplete around half her health bar, she will approach the priest and suck his blood like he was a ripe fruit. You can avoid that by drinking his blood earlier - but this may prevent you from obtaining the achievement for not attacking citizens.
Overall, this is an exhaustive fight. Try to use guns, heal yourself and avoid as many hits as you can. In time, you should master effective dodging combined with Blood Barrier. You might be able to deal major damage with short combos.
After she's dead, you have to watch one more scene. Take her to a grave, and the mission will be concluded.