What can I say - you need high levels. I used Yuna as a Gunner with catnip and Trigger Happy level 3. This meant that I killed Paragon in one move(whilst also bursting several blood vessels). I also had the other two as Dark knights simply because I needed them to take a lot of damage from Trema without dying. My basic strategy was to have Yuna continuously usinf Trigger happy whilst the other two took damage and used a hell of a lot of curing items.....oh and by the way - at the beginning of the battle use a soul spring (can be bribed from an oversouled basilisk) it will drain almost all of Trema's mp so you won't have to worry about his ultima spells! Phew. Trema's attacks will continually cast stop on your characters which can't be avoided , even with ribbon so remember to use softs instantly. I just kept at him and used megaelixers when necessary(which was often). I would recommend you keep your hp topped above 10,000 by equiping necessary armor. If he deala about 4000 damage to you each time then you know that you're not strong enough yet - when I beat him he only did a maximum of 1000 damage to my dark knights every attack - this allowed time to heal.

Keep at it with Yuna and you should eventually beat him - ps - dont try and hit him with anyone apart from your gunner because it is most likely they will miss and waste a turn.