Rash is fine honestly. Any nature that ups Sp.att (except quiet) is good for female espurr, because it will probably start to get 1 shoted at some point in the game (45-50+), but by then you will have access to gardevoir and/or alakazam.
Although to be perfectly honest, the pokemon that's helped me most (out of the psychic line) in reborn was deffinitely Delphox. It's a nice special sweeper having Good speed/sp.attack and if you want can teach it will'o'wisp so defense problems kinda solved (or reduced xD). Espurr is good but only on early stages of the game (lvl 40-45). After that it becomes inadequete. As for gardevoir.. It is a good pokemon. Deals tons of dmg honestly, but it has many more weaknesses due to it's fairy typing now, that against a toxicroak, for example, that would poison jab it, it might not even make it to kill the toxicroak :x. So that's why i think Delphox is the best.