There is no secret to completing this achievement in ordinary "legitimate" multiplayer. Find guns that work for you, find maps you can dominate in, pray the enemy players are garbage, then bust your ass. Player matches count, and that's the only fact you need that cannot simply be gleaned from the achievement description.
Though I must assume that because you are seeking a solution to this achievement, the patently obvious is not what you're after. It takes one hour for a single person to complete this achievement by boosting (remember that is per person, so e.g. a group of 6 will take 6 hours) at a rate of about 150-225 kills per ten minute turn. The large spread is due to the type of weapon you may be using while collecting this game's other achievements for kills with specific weapon types, since e.g. shotguns and snipers will get substantially less kills per turn than SMGs or assault rifles.
Get at least six people together, and from the game menu select Versus > Xbox Live > Host a match > Player match. You will see the Create Match screen. Here, modify these settings:
Game mode: Team deathmatch
Map: It makes no difference, most players are familiar with Villa though
Max players: 10
Private slots: 9
to enter the Advanced menu. On this screen, modify these settings:
Round duration: 10 minutes
Join in progress: No
Random respawn: No
Then start the match to enter the Game Lobby screen. You must change a couple more options that only appear here in the lobby. Select Match Settings on the left, then hit
to view the Advanced menu. Change these settings:
Lock Teams: No
Team respawn: Yes
At this point, you can invite everyone into the lobby.
You want to unbalance the teams so that whomever is getting their kills is killing a full group of 5 on the opposite team, so if your group is six people it will be 1 vs 5, if you have eight it's 3 vs 5, etc. The killer moves to the opposite spawn and takes the entire round to collect as many kills as they can, and everyone else must simply remain still. The enemy players will continue spawning in the same location, making it very easy to gun them down. This process is made a lot easier if each of the players being killed have to same weapon equipped, so that the killer can pick up ammunition from weapons lying on the floor.