The are 5 multiplayer maps, you need to win on each one of them.
These can be done in local multiplayer with two controller. Make sure to either put down some installations with the right trigger menu, or get in one of the vehicles on the map to kill some of the enemy soldiers that approach your toy box. Doing nothing risk the idle controller winning the match.
You'll win the match after enough of your soldiers reach the enemies toy box.
I imagine you can also get this on XBOX Live if you dont have a second controller.
EDIT: As kevgeotech notes the second controller need to be signed into an account, I was logged into one of my silver alt accounts on the second controller when I got it.