Yuna, Auron, and Rikku were the first party I uberlevelled. Of course they dealt 99999 easily.

I believe Lulu and Tidus come in next. Wakka's stuck doing about 2000xD if that much even. I dont' remember about kimarhi.

You could use an attribute sphere to build up stats, I believe. But it's easier just to remodel the sphere grid, and works better too.