Well save your game first. Then battle around the maze and have every Aeon reach overdrive. You can increase your aeons stats if you defeated Belgamine earlier.
Issaru uses his aeons in this order (ill give tips with it)
Ifrit (Grothia) – summon bahamut. one mega flare attack takes care of him.
Valefor (Pterya) – summon an aeon that can heal itself like ifrit, ixion or shiva. I recommend Ixion since Ixion has a high magic attack and defense. Use Ixions overdrive. The recovery time is high so if Valefor takes some major HP away cast thundara on Ixion. Afterwards change the attacks between Aerospark and regular Attacking
Bahamut (Spathi) – This one is a bit more tricky. I suggest Shiva. Bahamut will begin a countdown before he attacks with mega-flare. Use Shivas overdrive. Hopefully she will recover before the countdown reaches 0. Before it does, use shield to half the damage. Keep attacking and using heavenly strike and healing with Shivas own magic. The battle should be over after 3 overdrives or 2 are used.