At the start of the fight approach him just a bit and knife him in the legs, around the knees or so. You may need to enter a dodge prompt, so be ready for that. Once he kneels down, slash him in the head till he stands up. Repeat. Try to back him against a wall to keep him from moving much, also giving him less opportunities to attack, thus negating your need to enter dodge prompts. Done properly, he can be defeated in less than 30 seconds with this method.