Pterya (Wing in the PS3 version of Final Fantasy X HD Remaster) 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. Mega Flare will deal a large amount of damage, possibly even breaking the damage limit, and Bahamut is tough enough to withstand Pterya's normal attacks to refill his Overdrive gauge.