Blasphemous

You'll find this boss in the lowest and leftmost section of Mercy Dreams, fairly early into the game. What helped me the most-

Talking with Viridiana, just a little above the Prie Dieu before the boss door- she'll heal you during the fight when you're low

Getting in close when i could- fella does a long and low sweeping attack and i found that when hugging his ankles, i could swing a couple times when the animation started, parry his arm, and get a few more swings in afterwards

Dodging through him when he goes for the stomp attack

hope this helps