This boss is guarding the pagoda in Ashina Reservoir, the same pagoda where Kuro was initially kept during the prologue. You don’t need to clear all enemies on the way to him since you can fight him without aggroing anyone in the immediate area. To reach him quickly and bypassing several enemies along the way, start off from the Ashina Reservoir sculptor’s idol and jump over the wall to the left. You should be able to grapple on the branch there and to the other one overlooking the ledge past the gate.
Ignore the musketeer on the right; turn to the left immediately instead and kill the spotter before he makes any sound. You can now freely kill the musketeer behind you or ignore him.
There’s another musketeer behind the gatehouse. Sneak under the gatehouse to reach this musketeer to take him out. From there, sneak to backstab the other swordsman looking over the ridge.
Stealth Takedown is recommended to make this battle easier. If you use the off-road path where you'll shimmy and use the Grappling Hook to reach the pagoda, he'll immediately notice you. Carefully walk behind the wooden fence near the cliff instead to force him to move downstairs. Drop down and quickly grapple up so you can circle around him for a free stealth deathblow.
How to defeat this boss?
As mentioned above, landing a stealth deathblow is recommended. This boss’s massive spear has long reach making it hard to calculate the safe distance to avoid getting hit. He has several unblockable attacks that he’ll use inconsistently and unpredictably. You can only use your Mikiri Counter against his thrust attacks.
You can throw Oil to soak the boss with it then follow up by using Flame Vent combo to set him on fire. This can interrupt his attacks as well and allow you to land some free shots. Just don’t be too greedy with your strikes since the boss can easily recover and you can’t easily escape his range. Repeat this process until you defeat him.