Since I'll be going pretty in depth, I'll put the whole strategy in spoilers.

Preparing for the final battle shouldn't be too hard. The first time I played, I beat it on my second try, at levels 30-35. My second playthrough pegged the boss at levels 20-30, although I was doing strategic levelling up. Before going into the final battle, make sure everybody who you plan to fight with has 10 JP, Try to have as many people as you can with combo abilities and the 'Strikeback' R-ability. Before going in, check what the laws will be. Bring any law cards you might need.

Okay, first battle.
(SPOILER)Llednar shows up, flagged by some guys. All you have to do is beat Llednar, so focus right on him. Just use your best attacks and magic, and he should go down easy. For me, my ninja countered his physical attack, and Llednar went down in one hit. Not too hard, you shouldn't need to use totema or combos.

The second battle (SPOILER)is between Remedi and some Totema. Again, all you have to do is down Remedi. The only she can do is physically attack you, so Strikeback and Reflex are good abilities to have. Again, try to peg her with your best magic and physical attackers, and she'll hopefully go down before the totema can mess you up.

The third battle is the hardest by far. (SPOILER)Li-Grim can summon totema, which is essentially an instant game-over. Try to give your white magic user an angel ring, if you have one. I've given her the "slow" status before, so try using the ninja's earth veil. The strategy here is to have as many combo-laden characters as you can crowd around Li-grim, and then use combos to deal massive HP damage. If you don't have many combo users, use totema and your best magic.

If all else fails, just level up a lot, and you'll win eventually.