Grothia

Grothia is the name that Isaaru has given to his Fire-elemental Aeon, the one that is more commonly known as Ifrit. He is summoned to fight against Yuna in a battle of Aeons when the party is trying to escape the Via Purifico in Bevelle.

As two Aeons of the same type cannot fight against each other, Ifrit is out of the question (although it would be pointless as they would only heal each other). The best bet is to use Bahamut, however he will be more useful against the more powerful Pterya coming up next. Out of those remaining, the one with a clear advantage is Shiva, summoned with an Overdrive at the ready if possible. The added ice effect of Diamond Dust should be enough to destroy Grothia straight away, or leave him badly wounded in any case.

Should Shiva fail, summoning Valefor is the next best as Grothia's normal attack has a high chance of missing it's mark.