Sorry but that boss was incredibly easy for me. Rush the one that uses pyromancy, once he is dead they will pop the snake heads, then split them up and take one out at a time. If you hear one cast the giant snakes, just keep side stepping. Don't try to attack while the giant snake is out. Or you can be cheesy and just attack all 3 without killing them, but getting them to low health. They won't transform till one dies so you can get them all low on health, then take each of them out with a few strikes even when they transform.
I got the pyro guy down to a few hits and the other two transformed. I then finished him off, but couldn't get near the other two when they were snakified. Would you recommend rolling right up to them and getting a few hits in before dashing?
Play conservatively like you are fighting another human. Right from the fog gate I rush the fire one and get as much damage I can. Then I back up to the other side of the arena and the two katana guys will follow you but the pyro wont. Once you split the two from the pyro, go and rush the pyro. Repeat till pyro is dead, then copy the same mentality for the last 2. Once the pyro guy is down, it's pretty easy cause they only do physical attacks.
Yeah just beaten them, divide and conquer worked. Now having trouble with that bloody hunter in the house afterwards. You wear him down for ages then he just kills you with a lightning shower.