To get to War Stories, select your server, pass the initial loading screens, at the main menu select 'War Stories' (above 'Multiplayer'.
Before you can earn this achievement you need to complete a war story to unlock 'CHALLENGE MODE' and you will need to have a tier X tank of your own, researched and bought.
After you have both of the above, you complete this achievement by completing any chapter of the story you beat with your tier X tank.
NOTE: There seems to be a glicht where the game says you gained elite status for a tier X tank once you upgrade your Berliner or Emcha. This does not mean you unlocked that vehicle for use. (credit to Crater Bob)