Lone Shadow Masanaga the Spear-Bearer is an optional mini-boss in Sekiro: Shadows Die Twice, located in the Ashina Castle region.
From the Old Grave Sculptor’s Idol, turn back and kill the two enemies through the gates. Then turn right and head over the bridge towards the entrance to the Sunken Valley – a purple warrior will jump in and fight the two enemies that are waiting on the bridge, so let them all do battle and then finish off the one surviving enemy once the fight is done. Head through the gates and down the hill towards the entrance to the Sunken Valley, when you get to the bottom of the hill there will be three dogs waiting for you, so take these out from a safe distance with your Loaded Shuriken.
Now, creep through the house and up the steps in front of you but stick to the left hand side, this will stop the boss from spotting you and prevent the fight from triggering. When you get to the top of the steps you can eavesdrop on Lone Shadow Masanaga for some lore, but more importantly you can creep around the left side of the building to get behind him for a Deathblow, cutting the fight in half straight away.
Once that’s out of the way, the fight is pretty much a carbon copy of the Lone Shadow Longswordsman fight in the Ashina Reservoir. Like that fight, this one also takes place in a tight space, but we can finish it quickly if we know what we’re doing. Once you’ve hit that stealth Deathblow, Lone Shadow Masanaga will instantly use some Yashariku’s Sugar, which gives him a big damage boost but reduces his Vitality and Posture. When he’s using the Yashariku’s Sugar, use a Gokan’s Sugar of your own to reduce the damage done to your own Posture for a limited time. That should counter his attack boost and it should last long enough to get you through the fight. Once the fight is underway, Deflect all of his attacks and his Posture bar will start filling rapidly. Watch out for his Perilous/Red kanji attacks, one of which is a thrust kick which can be Mikiri Countered, while the other is a sweep attack that can be jumped over.
Keep at this and his Posture bar will fill rapidly. He does also perform an attack combo that fills your Poison meter with each hit, so Step Dodge backwards out of the way of these if you can. Don’t worry too much if he does manage to Poison you, because if you’re managing to Deflect his normal attacks, counter his Perilous attacks, and get your own hits in in-between then you should easily break his Posture and hit the deadly Deathblow well before the poison kills you. If he does poison you, just use an Antidote Powder to get rid of the effects.
Killing him will earn you some Yellow Gunpowder and a Prayer Bead, and you can loot a Dragon’s Blood Droplet from the body that he was stood next to.