SPOILER ALERT FOR THOSE WHO CARE

Spoiler:
Nothing special happens. You beat 'Inward Chaos' Chaos, who's like fifty times harder than the normal Chaos, and you get a basic Final Fantasy ending, going like "you are the chosen one," and whatever.

After that, you unlock the Genji equipment in the Shop for purchase, and the Blackjack course in the Duel Colosseum. Tough, that one.


END SPOILER ALERT, BECAUSE YOU REALLY CARE

I'd recommend using Onion Knight or Zidane for those tougher fights. They can evade many attacks, and have plenty of Link HP Attacks for versatility. I would wish you good luck, but no amount of luck in the world can save you. You need skill going in. But... good luck anyway, if you decide to try it again.