Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Why Soraka mid?
Whenever I am a Soraka support, I tend to play an aggressive Battle Mage style support. So why not take my aggression out on someone else? Soraka has pretty steady damage output, and the Rylai's helps with peel for your back line. The Athene's is to help pump out heals if necessary. Other than that, it is a pretty classic mid lane mage build. You ideally want 45% cdr to pump out your damage. If the enemy has more AP threats or Ad threats than vice versa, you could trade Zhonya's for Spirit Visage or Abyssal Scepter for something like Lich Bane. Just depends on your playstyle.