How do you kill Great Shinobi Owl in Sekiro: Shadows Die Twice? Towards the end of the game where you start making some big choices, you may end up fighting Owl, Wolf's foster father. Found atop Ashina Castle where you defeated Genichiro Ashina, he will be a very tough opponent, but at this point in the game, you should have the skills necessary to beat him.
Sekiro: Shadows Die Twice - How to beat Great Shinobi - Owl
This is a straight up blade on blade battle, but he does have a couple of dirty tricks up his sleeves. One attack sees him send out a charm that creates a smoke cloud. If you touch this smoke, you'll be unable to use any healing items for 30 seconds. When he does this, you should be able to dodge around him and deal some damage. Avoid the smoke as best you can so you can continue using the Healing Gourd.
He'll also throw two shuriken at you and follow up with a leaping sword strike. This should be sidestepped, and then you can counter with a few sword slashes. Other than that, you'll need to learn his attacks, deflect where you can, and keep on hammering away at him. Use Shinobi Firecrackers to stun him if needs be, and the Loaded Umbrella comes in handy with its easier deflects.
The second phase of the fight is largely the same, but he'll also try to poison you after his backflip attack, so make sure you have some Antidotes just in case. He'll also disappear into smoke. When he does this, the best thing to do is to jump away, as he's likely lining up an attack, and you won't know exactly where from. This is one of those boss fights where there's no quick trick to win -- you just have to gradually reduce his health to make his posture gauge fill more quickly.
Once you do defeat Great Shinobi Owl, you'll get the following: the Great Shinobi Memory, Aromatic Branch key item, and 6,000 XP.
You can also fight Owl at Hirata Estate, depending on choices you make during the course of the game. The fight takes place in the same area you fought Lady Butterfly, and the fight plays out very similarly to the above. Defeating Owl here will give you the Foster Father Memory and the Aromatic Flower key item.