Oooh those dark aeons. I've never managed to beat even one, not even Dark Valefor, which is the one who's meant to be the easiest?...
Using Yojimbo's overdrive is a strategy I've heard, but for some reason every time I summon my Yojimbo on a dark aeon, he auto attacks with wakisashi immediatly after being summoned. The turn goes over to the dark aeon, and the party of FFX is pretty much history. Bottom line; I fail.
Ok sorry you were asking for help and I just took your topic as a liberty to waste your time with my own dark problems.
(I love that your username is DarkYojimbo. I'll leave you alone to the advice of awesome people now. )