Riku didn't wipe the floor with Roxas as his Ansem form. It is stated in the game that Riku barely defeated Roxas, and was hardly able to bring Roxas to DiZ.

And I think you might be mistaken, or misinterpreting one of the cutscenes. Riku does not defeat Roxas in any of the KHII cutscenes.

In the scene when Roxas attacks Riku on the ground, and Riku strikes back, and the scene cuts to black, that was just Riku attempting to get away, despite the fact that Roxas falls back injured. If it were the case that this was when Roxas was defeated, Riku would have no need for his Ansem form.