To get this, you need to complete level 33.
Note For Hexic (Windows) players: If you have already done this requirement on the windows 8 version of the game, to get this to unlock, all you need to do is complete level 33 again on the phone version, assuming you're playing on the same save file via the windows Cloud. This means that you can get most of the Windows Phone 8 Hexic achievements out of order.