Sekiro: 10 Helpful Tips & Tricks To Defeat The Final Boss
9. Use Your Items
By this point in the game you should have accumulated a lot of items so make the most of them.
Sugars are especially useful (obviously not the stealth one) and you can consume 3 of each so that's 12 possible uses, but don't forget, only one at a time and the effects don't stack.
A possible tactic is to use Ako's Sugar for round 2 (the first round with Isshin) as you want to dispatch him quickly and with minimal health loss, then either Ungo's or Gokan's for the final two rounds.
Only use Yashariku's Sugar if you're confident as you take a huge cut to your health bar, but you do double the damage of Ako's Sugar.
You can also combine Ako's Sugar with Divine Confetti for added attack damage; you should have access to unlimited Confetti via the merchants as long as you have cash.
Don't forget about your other healing items like Pellets and various forms of rice in case you run out of gourds as well as persimmons for posture rejuvenation.
The Ceremonial Tanto will grant you 15 more spirit emblems and if you have Bundled Jizo Statues they will allow you to ressurect one more time.
There's no sense in hoarding at this stage.