Update

I FINALLY killed the bastard. I did it with Onion Knight. I found that you could semi-loop him into jumping into your attacks. I'm not sure if it's Turbo Hit or what(that aerial physical Bravery attack), and use Guiding Swipe forever until he dies. Sometimes, he'll escape the combo, but chase him and attack fast. Usually, Onion Knight hits so fast, they Chaos cannot activate any of his devastating attacks. I made it through all three phases without seeing a single Brink of Delusion! Of course, even with that, he still has a chance of cheaping you out. For example, those fire swords are still a bitch to dodge. Scarlet Rain is like... Ugh... But he almost never uses it, so... Only thing I can say is, for the first time for a final boss, disregard all strategy, and become reckless. That's what I did, and I won.

But even if you can't defeat him, don't worry about it. The reward is just Genji equipment in the shop, which isn't as good as it looks. -_- And, no, you can't get Shinryu, sadly. Still, I suppose its bragging rights, Ultimate Chaos is one hell of a bitch boss... o_O (Just so you know, ever since I beat him, I never challenged it again, I'm done with that FOREVER!!! That's the feeling you get.)