The easy way is to use Yojimbo's Zanmato.

The hard way (which is what i recomend) is to try leveling up.
You can try the Omega Ruins but it's not enough if you want to beat the Dark Aeons.
The other one (and the best imo) is the Don Tonberry trick.

Also it helps alot if you have the Celestial Weapons (especially Wakka's).