At the very beginning of the game I use Ifrit's AP to teach Shiva Refire Ice/Water magic, then kill the fishes on the beach outside Balamb. That will give you 20 waters per fin they drop, easily enough for 100 each... Give Ifrit to Squall and give 100 waters to Strength and his attack will make the GF look like a joke.

Not saying you should restart or some crazy, just giving advice through example. You just have to learn the Refine (and Card and Card Mod) and discover what turns into lots of spells, junction it to something most likely Strength, and off you go!

Alternately, you can just use summons over and over. Since leveling does nothing in this game you really shouldn't be fighting anybody but the necessary fights most of the time anyway, at least until you get Diablos's mug, which will give all the items you need for just about anything.

Just don't level too much too soon or issues can arise, as monsters level with you and can surpass you if you aren't careful. Card will give you AP but no EXP, essentially putting you "ahead" of the game. Once I have the airship and can run amok as I please I level without worrying about much anymore. I usually end up 100 before the end, but Squall's Lionheart limit break gives him a +50 to Sexy and eliminates the final boss with ease.