Sekiro: 10 Helpful Tips & Tricks To Defeat The Final Boss
2. Practice
This is going to be obvious and annoying to hear, especially if this is your 50th attempt, but if you're struggling with any particular aspect of the fight, go and visit other enemies who use similar attacks, and practice.
Genichiro is probably your best bet. Get to a stage where you can bait and anticipate his moves and react accordingly, especially that Mikiri counter.
It's all about finding rhythms and patterns.
If Isshin is charging his spear attack, you know you can jump on his head to avoid it. If he breaks out the lightning then jump, block, land, attack.
Don't forget the save point before the Divine Dragon either. You can practice returning the enemies' lightning attacks to really get this animation down. Ashina Castle and the Outskirts also have a lot of fast and heavily armoured enemies, plus an ogre.
All this practice helps, and you get XP and sen for levelling up, learning techniques and buying items like Confetti. It's a beautiful cycle.