Dodge his first attack, and instantly use the buster, he'll then swing with his left fist, as quickly as possible you must use your buster and hope the timings right, then he'll fall forward, do it to his head then to finish him.
You'll then get a scene during the credits where you have to protect Kyrie for 1mins and 30secs, doing this unlocks an extra ending sequence including Lady.
Credit to .Impact for the sig & avy