Nioh has been compared to the Dark Souls games, and as with Dark Souls, the bosses can be ridiculously tough to beat. Before you throw your controller out the window however, there is a simple glitch you can exploit to beat any boss in the game.
Here is what you do: power up your living weapon using the Soulstones (use a quick select item slot to do this). After buffing your weapon, use the Triangle+Circle key to spam it. After it is empty, do it again with another Soulstone. Save before doing any of this, just so you don’t lose all the Soulstones if you screw up the fight.
… And that’s it. It really is that easy.