This has always worked for me so here you go.
This battle really starts when you fight...I don't remember it's name but it's what Lord Zaon becomes. Heres how I did it.
All girls level 99. Have 1 girl Lady Luck, another a Thief and the last a Dark Knight. Have the Lady Luck and Dark Knight be equipped with the Higher Power Garment Grid and the Gunner on anything you like, really. On the Higher Power Garment Grid, have Dark Knight and Lady Luck right beside eachother and on the other Garment Grid of your choice have Gunner and Thief right beside eachother. Have Lady Luck equipped with a Speed Bracer and a Crystal Bangle. The Thief should have Cat n** and any other item that raises stats (I went with a Rabite's Foot for higher Luck). The Dark Knight should have a Crystal Bangle and Crystal Gloves. Your Thief will also need to have Trigger Happy Level 3 Mastered because she will become a Gunner.
Now in the first battle immediately have your Lady Luck use Bribe for 1,000,000 Gil. This will end the battle and give you Dark Matter(s). Hope to heaven that he doesn't kill your Lady Luck before she uses Bribe. If he does then you are dead.
When the battle with Trema begins, change your Lady Luck to Dark Knight. Have your Thief Pilfer MP until Trema cannot use Magic anymore. Only use the Dark Knights to heal and revive your party. Once you have pilfered all of his MP, change your Thief into a Gunner and use Trigger Happy. Eventually Trema will either kill the Gunner (at which time you would use a Phoenix Down or Mega Phoenix) or bring her down to low health. Then she will do 9999 with every attack in Trigger Happy. Try to always keep your Dark Knights at above half HP. Sometimes his physical attacks will cast Stop on the targeted girl. If it is used on a Dark Knight then don't worry about it but if it is on the Gunner then heal it right away. After a certain amount of damage is dealt he will use Meteor. Heal everyone with X Potions and Mega Potions ASAP. After more damage is dealt he will use Meteor again and attempt to use Ultima right after. But he won't be able to perform Ultima if you pilfered all of his MP. Heal your party accordingly and continue to use Trigger Happy. He should be dead in only a few more turns.
I found using Mega Phoenix was always better then Phoenix Down, just in case something goes horribly wrong. This battle still requires a lot of luck.
Good luck.
