Stealth-killing a single mob for 2104 exp and doing a quick reset. 12 seconds for the loop, resulting in ~10500 exp per minute.
Other Sekiro Guides:
- 100% Achievement / Trophy Guide.
- How to Defeat Gyoubu Masataka Oniwa (Boss Guide).
- How to Defeat Lady Butterfly (Boss Guide).
- How to Defeat Genichiro Ashina (Boss Guide).
- How to Defeat Folding Screen Monkeys (Boss Guide).
- How to Defeat Great Serpent (Boss Guide).
- How to Defeat Guardian Ape (Boss Guide).
- How to Defeat Great Shinobi - Owl (Boss Guide).
- How to Defeat Corrupted Monk (Boss Guide).
- How to Defeat Great Colored Carp (Boss Guide).
- How to Defeat Demon of Hatred (Boss Guide).
You need to be in a third-stage Ashina Castle, accessible after you kill the Divine Dragon.
You also need to have an Ashina Castle Upper Tower - Antechamber idol unlocked - this is the one right after you zip into the window of the big building that you later fight Genichiro on.
The Killing Part
Again, the idol to warp to: Ashina Castle, Upper Tower - Antechamber.
Go up the stairs and destroy the wooden/paper wall (is that what they call shōji?). There is a purple shinobi standing right behind it. Murder him. Resting on an idol will not reset the walls, they will still be broken, which is convenient.
And that's what you'll be doing: rest on a bonfi idol, then simply sprint to the shinobi, while getting slightly behind him, perform a stealth kill, repeat.
Bathe in Your Earnings
This will net you 2104 exp per kill if you're in NG+, and one cycle took me 12 seconds, so you're looking at ~10500 exp per minute.
I know you've always wanted to spend dozens of levels to acquire another weapon art that you're never gonna use anyway because it's hardly worth it at all. But hey, at least you'll get an achievement soon™. Go for it, skeleton, and onward to heightening your technique!