Well, for the most part, your aoens will develop as yuna develops.

My suggestion for leveling up in the sense of the game is, get the final weapons for atleast tidus, auron, and yuna, and the power up devices for them. By all this extra fighting you will gain lots more experience.

Also, a good peice of advice is to get all of tiduses overdrives. It's really simple. Just set his overdrive mode to slayer, go to some place, unleash his overdrive such as energy rain, kill all the monsters in one blow, and fill up you overdrive bar, mostly. If you have a weapon that has double or triple overdrive it will fill up all the way every time.
Do this over and over till you have it.

Last, capture monsters, you can get good items, for putting on armor to protect yourself in omega dungeon.