Defeating Tokichiro is what triggers your fight with Kashin Koji. As the fight is triggered, there is going to be a cutscene where Kashin Koji will present his monologue and after the cutscene ends, your fight with the Nioh 2 Kashin Koji Boss is going to begin.
Nioh 2 Kashin Koji Boss
Kashin does not possess a lot of armor but the attacks of the boss include all the elemental types of attacks. He does not have a large health bar so taking him down might not be that much of a big problem.
He swings his weapon around to hit you and since it is a long-ranged attack, it is best to move back from him at that moment.
Kashin also has the ability to bring out clones of himself. For these clones, all you have to do is hit them once and they vanish.
It is a very chaotic scene once they all come out as you are being bombarded with a lot of attacks at the same time. Just try to dodge them as much as you can and then hit them.
Kashin Koji tries to make elemental swords rain from the sky and you just have to block them as dodging them might seem a bit difficult.
Depleting Kashin’s health only once is not enough as he might fall down, he gets up again with his health bar full. You have to fight him the second time in order to take him out completely.
If you are unable to defeat Kashin Koji the second time and you die, you will have to start from the beginning of the first phase hence be careful.
Try to run in the big circle that surrounds Kashin as you can easily dodge his attacks here. The strategy is to wait for the opening of the circle, hit him with your weapon, move back and then repeat the steps.
In case you die, just jump in the hot spring, which is behind the third shrine, before you are ready to fight him again.
Once you defeat Kashin Koji, you are going to receive various equipment along with Smithing Text: Echoing Thunder. With this, you have completed the mission successfully and now you can end it.