You can find this boss in the main bridge of Senpou Temple, past the Shugendo Sculptor’s Idol, before reaching the Temple Grounds sculptor’s idol. You have to defeat him first before you can pass the bridge.
How to defeat this boss?
You can't damage the HP of this particular enemy so you have no choice but to keep attacking and deflecting his attacks to gradually deal posture damage to him. His attacks are slow and there will be a lot of opportunities to counterattack.
Using the Loaded Axe against him is recommended but you have to time your use right since you’ll still take damage if he catches you mid-swing. The important fact however, is that your loaded axe action won’t get interrupted and it will deal high damage to his posture.
Early in the battle, you also need to lure him near the walls of the bridge so he can destroy them with his attacks. Doing this is necessary to defeat him.
Once done, slowly build up his posture damage then make sure to position near the edge when you’re ready to land the deathblow. If done successfully, Sekiro will kick him off the edge, causing him to fall to his death.
* Prayer Bead * Breath of Nature: Shadow (Latent skill that recovers posture upon inflicting a successful deathblow)