I don't get it

So I'm currently trying to get 100% here but now i have a problem. Do i have to beat isshin in both endings to get his skills, or should i only get the ones that are unlockable for exp? I also bought some skills and got the one from Inner Genichiro, but do they count?...

Where? Why? I will defend you here. I simply want to know what boss or foible in the game caused you to stop playing. In fact, if you stopped playing and are reading this, you're the target of my question. If you stopped and picked it back up, I also want to know what caused you to quit, as well as what brought you back in. Don't be ashamed. The level of cognitive execution required to succeed in this game may be beyond you. It is made for healthy, young adults....

Or some way of making it easier? Since i always do terribly in these types of games and rely on coop to bail me out

how do i cheese this guy? i. can. NOT. deflect this guy for the LIFE OF ME. his health pool is MASSIVE. i'm forced to SLOG through a whole group of enemies every single attempt. he's like malenia on steroids and i have NEVER struggled this much with anything in a game before. THIS IS THE WORST BOSS EVER....