Both the keeper and the beast possesed soul have similar fire attacks, kinda a shot in the dark, but could the beast be an infected/transformed keeper?

This boss cannot be poisoned with just two knives. I just threw like 10. Nada. This does not work

You can chain an infinite number of parries on them. Parry them normally, as their getting up leap attack them and then IMMEDIATELY HIT THE PARRY BUTTON. The attack frames from their get up attacks occur almost instantaneously, so even if it doesn't look like they will get parried, they will. Then riposte them again, repeat for lulz. Found this out myself while fighting the one in Cursed Chalice.

- Despite the Bone Ash Hunters being female, the Keeper of the Old Lord's voice is male. Possibly an overlook by the developers?

Whirligig L2 fun locks the boss 100%

Use the normal form of Ludwig's Holy Blade and the Hunter's Blunderbuss. Back him into a corner from a visceral attack. As soon as the visceral attack is over, he will start to get up. Attack twice with the normal swing and take a step back. He will do his defensive two-hit combo where he moves backwards (into the wall). Between the first and second swing of this combo, shoot him with the blunderbuss for another visceral attack. Rinse and repeat. If you're on PSN, my name is UponABurn1ngBody

The shield negates almost all damage from his fire attacks

Its simple, just slash 4 hits with the Saw Cleaver then fire with the blunderbuss (make sure you have enough HP to survive a hit) and use a visceral attack. Make sure to rush in and start the 4 hits again with the cleaver and repeat until she dies.

This works for him and the Pthumerian Descendant. Two hand the axe and be a far enough distance away from him that his two fire slashes dont hit you. Wait until he slowly starts to walk toward you. Hold R2 and he gets anally destroyed, if you get hit you are almost guarenteed all your health back, AND he's knocked over. All it takes is some good timing.