Ujinari Mizuo is an optional mini boss in the game. He appears after the death of Isshin and is located in Isshin’s room. Defeat him and earn XP and prayer bead.
You can dominate Ujinari Mizuo by following the steps down there.
How to defeat Ujinari Mizuo:
Make your way into the room of Isshin, you will find him standing at the center of the room. Go outside, jump on the roof and then come back again.
Attack Ujinari Mizuo from his back and use the Great Feather Mist Raven for the fire effect because he is really afraid of the fire.
You can also use the Sazaku’s Lotus Umbrella for the fire effect, it will cause the same amount of damage.
And pack him with a final punch. Ujinari Mizuo is relatively an easy boss to defeat.