Pterya

Pterya is the second of Isaaru's Aeons that he summons in a battle with Yuna in the Via Purifico. It is the same Aeon as Valefor and uses the same attacks.

Since Valefor himself is not available, Bahamut is the path to take for this battle (and will probably be the first time he is summoned). Mega Flare will deals lots of damage (quite possibly even breaking the damage limit) and Bahamut is tough enough to withstand Pterya's normal attacks to refill his Overdrive gauge.

After Pterya has fallen, Isaaru saves the best for last: Spathi.