As of 12/14/17 USA, the DEV's are giving this troop out for free. Not sure for how long.
(Tynderi) Confirmed, got it by post. Achievement did unlock only after opening the mail, please note that the post has to be opened manually or otherwise it will expire and be deleted automatically (after certain amount of days).
(Ale M) Today is 12/18/17 you have 3 days left, and then the email will be auto deleted.
TA article 12/19/17. Earn Gems of War's Game Breaker Achievement Guaranteed for a Limited Time