The easiest way to beat this guy if you're going for 100%, piggy back off of what ever save you're using for Level 100 or if you're on Proud Mode with Level 100 Sora.
Equip Ars Arcanum and Ethers.
Keep spamming Ars Arcanum until his HP reaches 0.
You don't have to dodge, you don't have to pretty much give a single care about anything that is going on. Even if you whiff your attack, as long as you're doing Ars Arcanum, you will be invulnerable.
When you run out of MP, use an Ether.
You'll essentially beat this guy in less than 30 seconds.