At this stage of the game, it's not quite necessary to go all-out like Soulstone suggested. You don't really even need to change your party too much. Keep the Chocobo if you don't have any more humans that are any good, but otherwise replace it. The new human would be good as a chemist, priest, or maybe another archer. If you have enough money, buy Romanda Guns for your chemists.

The main thing to do in this battle is to stay spread out until you've killed the summoners. Don't keep more than 2 people near each other. Always keep moving towards the enemy, and take out the summoners first. After that, you can do pretty much whatever.