Achievement description is misleading. It actually unlocks for any Mastery badge, class 3 - Ace. (Credit to Kelle1994)
To earn a mastery badge you need to play a battle and score better than a certain percentage of all games played in that tank. It is calculated by base EXP.
Class III- better than 50%
Class II- better than 80%
Class I- better than 95%
Ace- better than 99%
To unlock this achievement you need to earn one of those medals in one of the mercenary nation tanks. These tanks can only be bought or earned through missions that take a while to complete.