Sekiro is a hard, hard game with nothing approximating an easy mode. The enemies are tough, the bosses are tougher and the final boss Isshin, the Sword Saint is, without a doubt, the hardest in the entire game, possibly one of the toughest in video game history.
Even FromSoftware veterans have found this particular foe a massive chore.
Given that most of you - if the percentage shown on the trophy list is anything to go by - aren't beating him, it seems appropriate to dispense some knowledge and wisdom in the hope of getting you at least one step closer to victory.
You can attempt to cheese Isshin by simply running in circles and baiting some of his attacks so you can nip in and execute a quick sword swipe. However, this takes forever, there's no guarantee of success as he can still catch you and deal massive damage, and worst of all, you won't leave feeling very satisfied.
To be frank, it's quicker and easier to learn and master a few moves, utilise the tools and items at your disposal, then learn to anticipate some of your foe's attack patterns.
So, if you haven't rage quit and deleted your file and you're prepared to put in the time, you can and will defeat this guy.