I never got the game (seeing as how I don't own a PS4 or 3 D:), but if I did I would grind the exp off him during flu, and then later have a fair fight using lots of persona. Personally, I would check the stats of my persona to make sure they are either offensive or defensive, and switch when I have to. Other than that, it's a war of time...and getting your ass handed to you a few time until you can figure out his pattern and what he'll do if something happens.