Mercenary tanks can only be bought with real money or earned through long missions.
This achievement is glitched - not a bad one, it may unlock after ONE battle is won instead of five. (Credit to Kelle1994 and Lord Terentius)
Once you have a mercenary tank, you need to find at least one more player with a mercenary tank to platoon with.
NOTE - if you join a platoon with one mecenary tank and one normal tank the achievement will not unlock!
Easy ways to find a partner.
1. Here on True Achievements you can create a gaming session. You can find this option on the right side menu of the game page about half down the menu.
2. The more relaxed method is to create Group on Xbox Live. To do this select the game options, go to "Game Hub", go to "Multiplayer" tab and either create the group or join an already existing one.
3. The last method would be to find someone during game play and sending them a message on Xbox live.