OK, so, this really is only for later on the game, once you have gotten Yojimbo. Jojimbo can do a move called Zanmato that automatically kills all enemies you are fighting against. However, it usually costs a lot of money (around 10,000 gil works for me) and on the occasion doesn't work if you don't use the Aeon a lot. Try to find the amount of gil that he uses the move most on and practice. This move is extremely good whith every boss including the last one where you can use it more than once, consecutively. Also use Zanmato on the Dark Aeons, it took me ages to figure out that I should Yojimbo.