This has been around since the JP version. If you are lv1, equipped with the EXP+20%/50%/100% accessories, wait for EXPx5 from the Chocobo, fight on your special day against a lv100 9/9 difficulty cpu, you can go from 1 to 100 in one attack. Exdeath is probably th easiest to do this against.
Let him land a BRV-attack so he gets 9999 brave, Magic Pot, land an attack. Ding, 2 million EXP. Done this with 5 chars so far.
YouTube - Dissidia FF: - Garland - Levels 1 to 100 - 2,000,000 EXP
Inward Chaos can eat shit.
They boost your basic accessories. It's kind of useless early in the game, but when you start getting Damage+15%/25% and EX-boosting items it becomes quite nice.