You'll get this when you complete the final level of the main campaign and beat the Boss Tank.
The level is fairly simple but here is how I did it:
- Drop a rock on the dynamite box in the lower left corner. This will cause the tank to fall and usually land upside down.
- Break the beige bar in the upper right corner that is holding 2 large boulders. The boulders will drop onto the tank.
- If the tank has any health left, throw rocks at it until it explodes.