If you don't feel like leveling up like crazy to defeat the Dark Aeons you can just use Yojimbo. Make him use his "one strike, one kill" move on them (beleive it's half your money +1 to get the best chance of getting that move). If you don't have Yojimbo, he can be found in the cave in the area between The Calm Lands and Mt. Gagazet.