This boss won't be that easy. Keep attacking it until it moves to the other side of the chamber. Then use the trigger command called standby. Then make Tidus use the cheer ability. Then Tros will attack both Tidus and the strange girl. Heal both of your characters with potions and then attack until he moves to the other side of the room again. Make the girl skip her turn with the triangle button and then use Tidus' Pincer attack if he has it.(It is a trigger command move) Keep this up and finally Tros will die. Remember to keep stealing throughout the battle.
