Most people got it on Headquarters, with 8 people, all with M36 SAWs. When someone died, used the fallen's ammo when you run out.

You need people that can be team players, use the drone well, and you need to shoot your M36 SAW on semiautomatic (giving you 800 accurately placed rounds).

Definitely attempt this achievement on the Headquarters map. Have at least five other people to play with and pack some M36 SAWs (note the auto-gunner class has the full 800 rounds, not other classes).

Now if you can tolerate it and have a pretty good size TV and extra controllers, go into the game with guests. These phantom players should equip M36 SAWs as well. When you go into the game, stash them behind the wooden shack to the left and deploy them when you need them. This helps when you need to get ammo but can't find any.

Lastly, it should go without saying, but make sure your online connection is solid. It blows to get 50 some minutes into the match and then your conection drops.