Low level means staying at the lowest level possible. It's possible to have most of your characters below level 10 at the end of the game. You don't get experience from boss fights, so you just have to run from every battle. There is some forced experience in some parts, but if you can force Gau to gain all of it then you can leap him off when you get new characters and his levels don't count towards the party average that determines their level.

Good luck with this. I'm currently trying a low level game whenever I have the time. I'm just at the Veldt in Sabin's scenario right now still getting items and GP, and hopefully a couple more Green Beret's before I'm done. It's pretty slow going since I'd like to stock up on 99 pheonix downs before leaving. I think I only have about 26 right now :rolleyes2.