Sekiro: 10 Helpful Tips & Tricks To Defeat The Final Boss
6. Dash To The Left
You may have noticed that during the first round with Isshin that he likes to sheath his sword (tee hee) and then do some form of fast and devastating attack.
Whatever move he does you can apply the same tactic.
Time it ('one Mississippi' should do) and dash to the left two times. This will put you behind him and cause him to miss you entirely, leaving himself open for an attack.
If he makes to do that charged attack just keep running to the left and move in closer as you do, you want to be standing close to his right hand side at the end. Again he should miss you completely and leave himself open for an attack.
It might take a little practice to get the timings right but, once you nail it, it becomes surprisingly simple and incredibly effective.