Quick and easy way to dispose of the Sanctuary Keeper. A lot of the preliminary stuff I did ended up being completely useless, as the Aeon made this a push-over.

What you need:
Yuna with Overdrive
Bahamut with Overdrive.

Summon Bahamut using Yuna's Overdrive (Grand Summon), and you'll get two shots at using Overdrive for Bahamut.