When the battle starts mash triangle to get through the reaction command stuff.
Afterwards when he's laying on the ground jump and do an X string in the air, jump again, and repeat.
When he gets up he will move backwards out of the stage and move forward on the ground moving his arms around. You can jump over the arms, go all the way back to the end of the stage, or the left and right corners to avoid the attack.
When the attack finishes his head will be on the ground, he will summon nobodies and shoot black and white string confetti at you. You can mash triangle and square at the same time to avoid most of the confetti, you can just run away from the nobodies he summoned and mash triangle, and you can kill the nobodies he summoned for extra health.
After he runs out of confetti he will stand up and shoot more out at you while moving around the field. You can move back some and press triangle to fly up and hopefully get some shots in his face, but if you are low on health I suggest finding one of the nobodies he sends out to get some extra HP.
He will more than likely repeat the triangle QTE again if you don't kill him after this point. You deal a fair amount of damage during and after the QTE so you could easily kill him while doing it a second time. There's a chance that he might go in the middle of the stage and charge up an attack while shooting confetti at you. For that I suggest running along the edges of the field while pressing triangle every now and then to avoid attacks so the charge attack doesn't kill you.