The beauty stat is in ORAS, I know, but is there a way to evolve Feebas in X without using the Prism Scale? I kind of don't want to have to trade it back to my ORAS copy to evolve it.
Unfortuneatly no, the only way to evolve Feebas in X/Y is to trade it while it's holding a Prism Scale.
Unless its Beautiful condition was raised to 170 or higher prior to being transferred to X/Y, in which case it will evolve when next leveled up.
Source: Bulbapedia and Serebii