Squall of Seed uses an excellent tactic. But I also have a suggestion for this. Allow me:

Use the same tactic like Squall said. But, when it comes the time to face Trema... instead of using items to make reflect or protection of magic... you can use also Soul Spring. With this you can absorb the Trema's MP and he'll not use his powerful magic. While you use your gunner with Cat Nip, the other 2 girls are on Dark Knight to do the healing.

SIDE NOTE: Trema will use only one magic that's called the Meteor. But your girls as Dark Knight will hold down this attack (because they have a high defense level...). Use the phoenix down to resurrect your Gunner to continue hitting Trema. If one of your Dark Knight girls is also down, you can use the mega-phoenix, to resurrect both...