This is attained on the last level. Change the difficulty to Easy before attempting this to make it easier for yourself. Throughout the last part of the game, your allies will kill almost all of your enemies. The only real situations that you'll have to deal with yourself are destroying the tank, and killing the Flamethrower.
When being attacked by the tank, aim for the two wheels. The more damage that is inflicted upon them, the more they turn red. After a few more shots, it will get destroyed. After dealing with the tank and moving to area that's covered, don't pick up the Minigun just yet. Wait until the waves of enemies are killed, and the Flamerthrower shows up. When dealing with the Flamethrower and his posse, you can wait until he gets close and use the Minigun on him.