they call me Varanus
What's a "Syncher"?
A Syncher is a Pokemon with Synchronize as its ability, such as Ralts or Espeon/Umbreon. Ralts is the easiest one to get in XY. If you have a Pokemon with Synchronize as its ability as the first Pokemon in your party, the Pokemon you are battling will have a 50% chance of having the same nature as your Syncher.
So for example, if you want a Modest Xerneas, you get a Modest Ralts with Synchronize and send it out against Xerneas. Xerneas will then have a 50% chance of having a Modest nature when you catch it, making getting the natures and IV combinations that you want a LOT easier.