There will be many ways to get this, earlier and/or easier, but I just got it on the 4th level of the campaign, Chopper Suey.
You start from the left side of the map. Walk towards the edge, just above the helicopter. Hold and charge the shot from your Bazooka until it touches the crosshair and release. With the Oil Drums and Mines surrounding the worms, you should easily get 4 kills.