Blood Run begins as soon as you complete "The Beast of Toussaint", and you'll soon find yourself face to face with the Beast of Beauclair, who asks how many innocents Geralt has cut down (but truly the answer here does not matter). What does matter is the boss fight itself, where Geralt is forced into combat with a powerful vampire, but you might be interested to know that you can win or lose this fight, as the outcome remains the same. Regardless, if you want to feel proud in victory, you can use the boss guide below!
How to Beat Dettlaff, the Beast of Beauclair in The Witcher 3
If you beat the Beast of Beauclair, you will earn 1,000 XP. However, if you lose the fight, you earn no XP, but the cutscene after remains the same, so we recommend creating a manual save before the fight so you can try again to avoid missing out on those sweet XP gains!
The Beast of Beauclair is weak to Quen, Black Blood, and Vampire Oil, so you might want to drink some Black Blood and coat your silver sword in Vampire Oil before you get into the fight, as these will help you fight the creature. Quen is also great for blocking some of the Beast's attacks, as he's quite fast and teleports around to try and throw you off your game.
Patience is a virtue with the Beast, so hold L2/LT to block his attacks while he's teleporting around, and while your Quen shield is up, try to maximize the number of dodges you take, as the Beast will try to teleport behind you and backstab you. You can counter this by dodging his teleporting and following with a flurry of quick attacks (Square/X).
The Beast has a lot of health, so it might feel a tad repetitive as you wait for his attack pattern to conclude so you can land a few quick hits. You shouldn't attempt heavy attacks, as he is quite fast! Once his health bar reaches around 5%, a cutscene will play, and this cutscene remains the same regardless of whether you win or lose.
At the end of the mission, Geralt will claim he has to investigate the crime scene, but this is just a lie to get the knights of Toussaint to leave. You can then either proceed with some side quests or go straight to the next main quest, La Cage au Fou.
Source: Game Rant