Gyoubu Oniwa is the final enemy boss in the Ashina outskirts. He will be riding a horse and the name of his horse is Onikage. He is known as the horse boss in the Sekiro game. You can kill him by using the Shinobi Firecracker tool.
Follow the steps below to kill Gyoubu Oniwa boss.
How to defeat Gyoubu Oniwa:
Climb up the tower with the help of a rope and then jump over the castle to reach on to the other side.
You will find Gyoubu Oniwa riding on his horse. Attack him straight away and try to scare jos horse by using the firecrackers.
Reach closer to him and jump onto the horse. And then slash him with the deadly strikes from the back.
He will come again in the second phase. You have to follow the same tactics, just attack him with the sword and fire crackers. And you will be able to beat him with just a few firecrackers.
Jump on the horse and kill him one more time with a deadly blow into his body.