The second Aeon that Isaaru summons is called Pterya, which is Valefor. If your Bahamut is still good after the previous battle, you can use him again here, but if not, Ixion, Ifrit and Shiva are good candidates. Pterya doesn’t have an elemental affiliation, so you don’t have to worry about accidentally healing her. The upside about using an elemental Aeon yourself is that you can use their respective element to heal them, like Shiva using Blizzara to heal herself. As far as what you’ll be dealing with, Pterya has her normal attack and Sonic Wings, which can delay your turn sometimes. There’s nothing you can do about this, but accept it. When her Overdrive is full, she will use Energy Ray, so make sure to Shield when you know it’s coming.