The only character in the party that I found to be absolutely worthless was Rikku. Tidus should focus on buffs, Kimahri buffs and white magic, Yuna on white magic, Lulu on black magic, and Auron and Wakka on offensive skills. That's what won me the game. The only black magic I really gave Yuna was Ultima, which was mostly important for the final battle of the game, since she and Lulu have the most MP for that sort of thing.