I've been boosting with people, and it seems that there's a lot of confusion about how best to do this, so to clear it up here's a step-by-step.
First, if you're reading this: ***STOP!!!***
*All TEAM DEATHMATCH kills count for regular deathmatch kills. Don't do regular deathmatch... you'll waste your time.
*You CAN boost this with just 2 players. Team deathmatch allows 1-player teams. That may also be easier, since Prey players are virtually nonexistent.
*You do NOT need to go into Ranked for this achievement. Get your ranked matches done first (which will help pad your kill count before you get here), and then come back here to mop up your numbers.
*Pulling the RT right after you die makes you respawn immediately, which in turn makes the boosting faster.
Now, on to the specifics.
*Party chat is intensely helpful.
*Set up for 100 frag limit, no time limit.
*Roadhouse is definitely the easiest map.
*For your weapon kills, the area by the bar is probably the easiest place. It's a good, large room where you can see people coming, and it has a lot of different ammo and guns nearby.
**Once you have all your weapon kills, probably the easiest place to set up a nice kill zone is in the basement next to the Leech Gun recharge station. Stand in the corner away from the portal. Your partner(s) can jump through the portal in the bar, come in from outside through the door, or come through the basement. The recharge station is a great landmark. It also lets you reload without moving.
If you've done the ranked matches first, you may not need any more kills after your weapon kills.
If you're not familiar with Roadhouse, take a look around when you start so you can find the quick ways to the bar and to the recharge station (remember, the portal in the bar goes right to the recharge station).