When first fought, the player is likely to have only the basic characters available (Tyro, Cloud, White Mage, Black Mage), which in itself is not an issue. Nor is Gravity. The biggest threat from Ammes is actually its accompanying Sinscales, whose Spines pack a wallop. Take a page from the source game and have your fighter(s) fell two of them while the Mages concentrate on healing the party and blasting Ammes itself, as the Sinscales are slower to respawn unless all three are gone. Two or three rounds of combat should be enough to take it down.
The player will get Wakka soon enough after this, and longtime players will probably also have Auron, Kimahri, Yuna or Tidus aboard as well, making this battle a good starting point to grind them using Record Synergy bonuses should their levels be low.