I have a super great shiny greninja and I want him to use the ash greninja ability, HOW DO I DO IT PLEASEEEE! :)
The only way to do this is to transfer the Greninja you used in the Demo of SM into your actual Sun or Moon Game.
If you are asking if you can turn a normal Greninja, not from the demo, into Ash-Greninja, you can't. There's no way.
What JasonE said is Right, a Greninja With Protean or Torrent can't become Ash-Greninja. You have to have a Greninja With the Ability Battle Bond from the Pokemon S&M Special Demo.