Snafu...I love you for making this thread.
Anyway, this all really depends on what version of Rikku this discusses. If it's FFX Riku, she'll most likely lose despite having multiple bombs and grenades at her disposal. Yuffie can run up the sides of buildings and is fast enough to avoid getting bombarded by those, and once Rikku gets hit with her large shuriken she's down for the count, and I doubt Rikku's that fast as of FFX. FFX-2 Rikku is much better since she can be a variety of different types of fighters. She can be a White Mage, Black Mage, Alchemist, Warrior, Black Night or she could use her ultimate dress sphere, the Machina Maw, that would surely be enough to defeat Yuffie. I think one of the best dress spheres to use against someone like Yuffie is the Black Mage dress sphere. Any third degree spell would kill Yuffie. FF7 Yuffie and KH2 Yuffie demonstrated equal power and skill, so it all depends what version of Rikku this is.