I managed to finish a few days ago. Yes, Ultimate Chaos is an a*****e.
Strategy on beating him is obviously going to vary depending on which character you use. Whoever you play, though, learning to dodge his attacks is critical. The two biggest ones I had trouble with are Divine Punishment and Soul of Oblivion.
For the former, dodge forward or backward to avoid the flaming pillars then left or right to avoid the following sword strike; if you botch this, you're getting hurt. Bad. The upside is Chaos is open to a counterattack for a second or two if he misses.
SoO is almost guaranteed to hit if it goes off, but it takes a couple seconds for Chaos to charge the attack, so dash at him and hit him while he's powering up and vulnerable.
Make sure you're at Level 100 (obviously) and have the best equipment you can get for your character. Accessories and equips that help you recover Bravery and EX Force quickly like Bravery Elemental and Close to You are great; being able to return to base Bravery faster means Chaos has less chance to Break you, and EX Mode can save you if you get caught by Divine Punishment (activate it just before the HP part of the attack hits.) Also, if you hit Chaos with an HP attack while in EX Mode, try not to use your EX Burst unless it'll finish him off; any extra EX Force you have saved up will carry over to the next battle.
For summons, Magic Pot is good if you're getting stomped by Chaos' Bravery attacks, but I prefer Carbuncle, Omega, or Odin for the defensive aspect (you need to get a little lucky with Odin, though.) Use your summon only as a last resort, though - you only get to use it once during the three battles.
Honestly, unless you're some sort of gaming god, beating Ultimate Chaos depends quite a bit on luck. I fought him somewhere around 15 times in a row before I got him. Hope he uses Soul of Oblivion a lot.