Crash Commando

For this you need to be on a map with a stationary gun. When you mount it, it will change the screen to another part of the map. You would obviously need four opponents within your shooting area to get this. As opponents will never stand there normally, especially not four in a row, you will have to boost this. (Click here for the boosting thread)

Play a round of death-match, have your four boosting partner(s) stand in the proper area, mount the stationary gun and gun them all down in rapid succession.

Easily gotten in the Tower level. If you spawn on the side with the turret just run/float to it and aim to the right. The other team spawns and runs right into your cross-hairs. Easy multikill. If you don't spawn on that side just switch teams. (Thanks to immikeulate for this paragraph)