Mercenary tanks can only be bought with real money or earned through long contracts.
Once you do have the tank, simply play the game and the wins will start to add up but be patient as this achievement could take some time. Of course if you have more than one mercenary tank, you could hop into the other if you die early in a match. This will make the process a bit quicker.
Along the way, if you play well, you should earn;
And if you play even better and a tier V or higher, it is possible to earn;