Just beat him this morning, it's pretty simple if you know what to do. After dodging his first wave of spells, get within melee range to trigger his swipe. If you time your gunshots with the apex of his swing, you can consistently parry/riposte his entire first phase by staying close to him.
After he buffs himself, he gains a new move set and becomes very difficult, if not impossible, to parry. For the rest of the fight, I recommend backpedaling away from him along the length of the roof, baiting his jumping attack. Stay locked on, and dodge forward-right just as he's coming down; he should land harmlessly behind you. His recovery after this attack is very long, so you should have plenty of time to fully charge your R2 from behind. V-attack, then back away. Repeat this process until he is dead.
Don't get greedy and try R1 spamming when he's down. After he buffs, he cannot be interrupted and he can combo you down in one go if you're not careful. Take your time and only hit him with charge and V-attacks for the second half of the fight, and you should be fine.