Trema is on Floor 100 of Via Infinito. You will fight Paragon and Trema back-to-back with no chance to save between, so you need to be prepared. Once you reach the 100th floor, immediately teleport and save before the fight. You can come straight back again.
My party: three maxed out Mascots on the Higher Power garment grid, Paine and Yuna with Crystal Bangles and Adamantite (or whatever else strengthens your defense) and Rikku with Cat's Nip and Rabite's Foot. Make sure you have a full supply of phoenix downs.
You will get a Cat's Nip from beating one of the earlier bosses in Via Infinito. You can get a Rabite's Foot by beating Jumbo Cactuar. Adamantite is available in the Gunner's Gauntlet as the top prize. You can get another with the Fiend Tales starting with the Killer Hound in Fiend Arena.
If you don't have good defensive equipment, you could use the Valiant Lustre grid instead. Put Yuna and Paine through the grid as quickly as you can to beef up both physical and magical defense. It's unnecessary with Rikku because we need her to die. Just remember to put Yuna and Paine through the grid again when the Trema fight starts.
The easiest way I've found to handle this fight is to first oversoul Paragon. He is very easy to beat this way because he spends most of his time attacking your people one at a time. Once Rikku goes down, bring her back with a phoenix down so that she's in critical health. The Cat's Nip will berserk her, and she will hit for 9999 every turn.
Once Paragon goes down, then it's Trema's turn, and this fight is all about staying alive. Have Yuna and Paine heal each other and phoenix down Rikku whenever she goes down so that her health stays in the yellow. Now just let her kill him. The Rabite's Foot will ensure that she hits him (without it, she misses most of the time); it also seems to make her the target less often.
Trema has a million HP (literally). You can tell how you're doing by his attacks. When he uses Meteor, you've knocked him down by half. He'll use it again at 250K. When he casts Ultima, you're almost there.