Whoa.
In this game,
LEVELS DO NOT MATTER. Whoever suggests you be at a certain level before fighting an enemy is dead wrong and does not know what they are talking about. You can, and should, go through the game by gaining the LEAST LEVELS POSSIBLE. Not only does this make enemies weak, but it also opens a big door later on in the game after you get the Ragnarok.
What matters in this game is the Junction System. If you know how to use it, then you will be able to totally annihilate every enemy at any level, be it 7 or 100. Early on in the game, make sure to Draw 300 of every new magic. Once you get the GF item refining abilities, you will be able to refine very powerful magic from items early on in the game, so having to level up to draw them becomes highly unneccesary.
Come fights such as Diablos who have useful magic that can be drawn, it is essential that you have a high Magic stat. This is the key to drawing 9 spells every turn. Once you have a good spell Junctioned to your Magic stat, drawing should become much easier. You should also make sure not to neglect your Spirit and Vitality stats. Your Spirit stat determines how much damage magical attacks do, and your Vitality stat determines how much damage physical attacks do. Make sure to Junction strong magic to each of those stats as well.
Playing the Triple Triad card game makes the game MUCH easier. If you play alot of games, then you can use Quezacotl's Card Mod ability to mod cards into items that can be further refined into magic. This is how you can get poweful magic like Curaga, Meltdown, Pain, Break, Tornado, etc. early on in the game. I had spells like that before I fought Edea at the end of Disk 1. You can also mod cards into items needed for upgrading your weapons. Here is an awesome card walkthrough:
http://db.gamefaqs.com/console/psx/f...ii_cards_d.txt.
Now for what you do when you get the Rangarok in Disk 3. Your Levels should be at a maximum of 20. Go around and get the remaining GFs like Bahamut, Eden, Cactaur, etc. Once you have them all go to the Island Closest to Hell. Kill two of your characters. Equip the character who is alive with a GF that has the Abilityx4 skill, and then equip 4 of the 5 Bonues abilities (Str, Mag, HP, Spr, Vit). Next, Level that character all the way to 100 with this same setup. Each of your stats that had a Bonus ability equipped will have increased by 80 and your HP by 2400. After this it will be very easy to max out your stats at 255 and your HP at 9999.
Right now in my playthrough of the game, none of my characters have gained ONE level except for Squall, who is at Level 8. I'm mid-way through Disk 2 and I have just gotten Pandemona from Fujin.