You need two or more controllers to get this.
Choose M1E1 in local deathmatch with two or more players. Set the kills to 25 and go around killing player 2/3/4 as player 1 until you reach 25 kills. There are only about 5 spawn locations so you may get lucky with respawns. You need to press fire to respawn the other players. Just do 4 lots of 25 kills to get this trophy. It should only take about 30 mins.