Snivy's hidden ability is unreleased, the only way to get it is by hacking (as of today.)
See how "Contrary" is crossed out? That means the ability is unreleased, so it can't be legally obtained.
It's possible, but hard.
The only way to get one is by hacking for one because it was never released. So try to hack or try to find someone who hacked for one.
Sorry for the sad news.