I've heard of people taking on the Dark Aeons with ridiculously low stats using various strange methods. Tav would be able to go into greater detail with that.

As to increasing stats if you want to go down the traditional route then you'll have to use the Monster Arena. For now (and I do mean for now since it takes AGES) I'd concentrate on capturing at least five of each monster in each area, preferrably ten, to unlock all the necessary monsters needed to level-up your characters sufficiently.