Actually, I'm dominating as a level 100 Sephiroth as it stands, I think I've been beat once by Chaos in the Duel Tournament mode, and that's because I wasn't paying attention, it doesn't get all that much more difficult really, especially once you get used to the character's movesets.
I dislike Sephy's Alt. Costume, I mean, I expected that it was gonna be that, but it still felt like a letdown, I still take him in his coat over the coatless version anyday, honestly, my favourite Alt. Is Cloud's one, I love his Advent Children costume, then you've got Squall's SeeD one, which wins major points, and I like Kuja's one too, though it's basic.
Try some new characters, Kuja and Sephy get my instant vote for best characters of that game, though I'm a little biased, considering I've always loved Kuja, he's very strong if you know how to use him right, though it takes some getting used to.