Pterya is the second of Isaaru's aeons that he summons in a battle with Yuna in the Via Purifico. She is the same aeon as Valefor and uses the same attacks. After Pterya has fallen, Isaaru saves the best for last: Spathi.
Since Valefor herself 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 deal lots of damage, possibly even breaking the damage limit, and Bahamut is tough enough to withstand Pterya's normal attacks to refill his Overdrive gauge.