There is a way.
Although it's not legit. There is absolutely no way you can ever get a Shiny Xerneas/Yveltal/Zygarde.
It's possible using Power Saves though, although you can't trade it after you get it, so you won't find any of those in the GTS.
NOTE: The usual process of Shinification (a form of hack) doesn't work, as the Pokemon are Shiny-locked.
Hope I helped!