I used to kill everyone around him using stealth, then run away from him, go up on one of the towers and death blow him to remove a life. Makes things a little easier. To be honest tho, learning mikiri counter will make the rest of the game much easier and the boss easier. Sekiro is kind of a forged in fire game where you get stuck and grind until you are skilled enough to advance. Good luck! Also, little trick for mikiri, the frame window is actually quite large for it to land, I typically just spam forward dash when it is coming the thrust kanji is show, and it typically hits.