There is a way of quick levelling that I used many years ago.

It requires an analog controller with turbo buttons.

There's a place on the island where Mideel is where, if you keep walking straight into the edge of the island, you run back and forth. If you use a rubber band to keep your analog stick going in one direction, you just keep running without stopping.

Then you set Circle to a turbo button. This makes you characters automatically perform their melee attacks when they're in battle.

On Mideel, you fight these big eel things that give decent XP, AP, and Turbo Ethers or something like that. Set yourself up like that, leave your game playing all night, and by morning you'll probably be maxed out.

You could probably use tape on the D-pad instead of a rubber band on the analog stick, and tape on the circle button rather than a turbo button, but that's they way I did it.

It's cheap, yes, but very effective.