I suggest boosting this with 4 players as it goes by much quicker. Start a deathmatch on Campus with running speed set to 1.5x, points to win at 0 and time limit 30minutes. Have everyone start with RPL sub-machine gun then run to the central area right in the middle with the low roof and one wall being windows. Each person take turns getting 50 kills against the other 3. The three dying just keep lining up against the wall opposite of the current killer and rack up the kills. Since everyone should be spawning with the same weapon, no issues with running out of ammo should occur. just pick up the fallen's ammo right in front of you. All 4 players will have no problem getting 50 kills in one 30 minute match. this has been tested and proven effective
As a side note, you can even multi task by having 2 people spawn with one weapon and the other 2 guys spawn with a different one. that way, you can work on two weapon cheevies in one game and not have to worry about running out of ammo! just pick up your partner with the same gun's ammo whenever you're out.