Army of TWO The Devils Cartel

You will probably get this during normal solo play just by completing the story as MMG placements are fairly abundant through the game but if you played co-op like myself and your partner was a MMG hogger just load up solo Narco Hell: Rooftop Rampage .

The level is very short just choose to get in the chopper and run the level till the end (around 3 mins per playthrough) and you should get 20-30 kills per runthrough.

Hope this helps :3

I found it much easier to acquire this achievement by using the Helicopter machine gun. Whenever the option is available choose to provide air support. Chapter 43 Fish in a barrel is a great place to rack up kills just keep restarting the checkpoint and you will have this in a few minutes. This is also a great place to earn Overkill kills as well, whether single or double