Regarding a certain rule set which allows you to start 1on1 battles with 9999 bravery against level 100 opponents, once you have unlocked Chaos as an opponent use the aforementioned rule set, against Chaos (sadly you cant change it AI type to calm), equip Beckoning cat and another ap, pp, gil, exp enhancing accsessories to you character. Make sure his hp attack is something mid or long range that starts off quick, Leon's Blasting Zone, Kuja's Ultima, Most of Prishe's attacks are perfect examples. Turn your play plan to Grinder. Its best to do this during a day with bonuses or the Special day the massive amounts of boost really stack up especially since its a one button battle, Chaos might dodge but in a few seconds either you or him will be defeated, if you land a single hit which should kill him you'll get massive amounts of exp like how it took 5 battles to raise my lvl 12 Leon to level100.