Crystal Clash

To unlock this go to the Shop and click the Redeem Bonus Code. You and a buddy can refer each other by putting in their Friend ID. The Friend IDs can be found at the lower right hand corner of the screen and clicking on the person icon. Then click on Refer a Friend to get your code. Copy and paste or type out the numbers in the redeem Bonus Code. Once done hit enter for an achievement.

Below is my code for Crystal Clash when put in you should receive 1000 Crystals which I recommend being spent on a skin for another achievement.

5257360

There is also eight secret achievements to add as well. There are guides to each one if you wish to try and find them by yourself.