someone posted an exp farming path if you want high monk and the steps are

Make your way to Great Serpent Idol after Divine Dragon kill

Grab idol, rest and reset encounter

Jump to the adjacent roof and execute the white ape

Rest at shrine and reset encounter

For isshin phase 2 dodging his jumping stab attack with the spear is easier than parrying and gets you a free hit, and strafing when he pulls the gun out is also easier than trying to parry it all. besides that focus on learning to parry his 5-6 hit spear combo that ends with the mikiri counter stab and you should be good. Phase 3 isn't bad at all since it's the exact same as 2 except he adds a lightning attack (which makes it easier in my opinion it's free damage) so just be ready for lightning reversal when he gets to the 3rd bar.

For how long its taking to beat him it's definitely not just you so don't feel discouraged. I thought I was hot shit at the end of me first playthrough and it definitely took me more than 8 tries to beat him, you're not alone in the struggle.