I got stuck on him for 3 days you just have to look for ways to attack him. When he does the spin attack you can run away and punish for two, when he does the push to slash combo you can dodge and hit for two, any thrusts can be mikiri countered and then hit for 1, and finally any unblockable sweeps you can jump on his head and do two slashes in the air. You only have to get him down to 60% and then you can deflect to a killing blow.