Headless is an optional mini-boss found in a number of places in Sekiro: Shadows Die Twice. You’ve likely already tried and failed to kill a Headless early on in the game in the Ashina Outskirts, and then you’ve probably done similar in the Hidden Forest area of the Ashina Depths.
This guide has been written specifically for the Headless battle in the Hidden Forest area of the Ashina Depths, but the same tactics can be applied to every Headless boss fight other than those that are underwater. Completing the Ashina Depths should gear you up perfectly for taking on Headless, so I personally recommend taking on this boss fight (and any other Headless fights) once you have completed the Ashina Depths, around the same time that you should be taking on the Shichimen Warrior fight in the Ashina Depths.
The reason that you should be taking these two fights on at around the same time is because they are very similar. Both are apparition type enemies, so your preparation for each fight should be the same.
The first thing to make sure you have is Divine Confetti, as this will make the fight much easier and quicker. You should have a fair amount in your inventory by this point in the game, but if you don’t have any then it’s worth trying to find some before you try the boss. The interior of the Ashina Castle is a good place to farm Divine Confetti, but if all of the Sculptor’s Idols there are blanked out at this stage of your game then there’s a good chance you missed a secret room in the Hirata Estate. To check, head to the Audience Chamber Sculptor’s Idol and head back towards the main entrance into the building which Juzou the Drunkard was guarding earlier in the game. Before you exit the building, turn right and follow the corridor around, kill the two enemies, and then hug the wall at the end. The wall will spin around and you’ll be in a secret room where you can find a few Divine Confetti. If you can’t find any Divine Confetti then the other items will still help you and you can still follow these tips, but it will take a lot longer.
The other items that you need will require a fair amount of Sen, so you might need to go out and farm some money to acquire them. You’ve likely already got the first item that you need, as it’s purchased from the Memorial Mob merchant by the Mibu Village Sculptor’s Idol. It’s called the Mottled Purple Gourd, and using it both reduces Terror buildup and increases Terror Resistance, which makes the boss fight much easier. If you don’t already have it, go and buy it.
The final piece of the puzzle is the Phoenix’s Lilac Umbrella, which is a Prosthetic Tool Upgrade for the Loaded Umbrella. Using the Phoenix’s Lilac Umbrella will protect you from attacks by apparition type enemies, which is perfect for blocking Headless’ attacks. To buy the upgrade you’ll need 1000 Sen, 6 Scrap Iron, 4 Lump of Fat Wax, and 4 Scrap Magnetite. You’ve likely got all of those at this stage of the game anyway, but if you don’t, it’s time to go farming. Make sure you have plenty of Spirit Emblems too, because you’re going to have to use the Umbrella a fair bit in this fight. If you don’t have any, buy some at a Sculptor’s Idol.
Now that you’re prepared, it’s time to take on the boss. Much like the Shichimen Warrior fight, before you trigger the boss fight you need to make sure that your Phoenix’s Lilac Umbrella is equipped, take a swig from your Mottled Purple Gourd, and then use some Divine Confetti. Then, charge in and go aggressive, and you’ll get a few hits in for some serious damage straight away. When Headless attacks, you can’t Deflect it because it’ll still cause significant Terror Damage. Instead, just use your Phoenix’s Lilac Umbrella to completely protect yourself from his attacks and then go back on the offensive. This fight is all about just blocking with your Umbrella and then being aggressive.
Keep at this and you’ll quickly break his Posture and open him up for the first Deathblow.
Thankfully, the second phase is identical to the first, so follow the same tactics and you should drop him in rapid time.
Killing this Headless will earn you Gachiin’s Spiritfall, which consumes Spirit Emblems to enhance Sekiro’s stealth capabilities. Killing other Headless bosses will earn you other variations of Spiritfall items which provide different benefits, such as taking reduced Posture damage for a limited time.