Online (gold) and at least two people required. Can be done in any deathmatch mode, player or ranked. Match must finish for the kills to count, so set a limit on frags or time. Some have reported randomly being booted from the game, so I recommend a frag limit of 25 for boosters, and a time limit of 5 minutes-ten minutes for non-boosters. Press RT to respawn instantly in-game.
There are still a few people playing this online, but they're all really good. The best map to play someone better than you on is E2M8. The high number of rockets makes this map very swingy. Skill won't matter as much. It's small size and large supply of power weapons makes it okay for boosting as well, if you can communicate well or have a four-person party.
If you're playing someone worse than you randomly, I'd prefer E4M1. The shotgun and the chaingun are powerful, but take some skill, and the map's rocket launcher is hard to find (jump in the first damaging pit, face the side with the clip of bullets, use) Also, The map is small enough to easily locate an opponent, but complex enough to confuse. If you have a group going for the achievement by playing together legitimately, try this level or E2M9.
The least complicated map is E3M8. It's a 4-point star with a line. Although the rocket launcher and plasma rifle appear, neither has much ammo, so legit matches usually turn into pistol contests. However, if you're having communication trouble in your boosting session, or only two people in your party, I'd still recommend it. One player should sit on the plasma rifle spawn in the center of the map, the other should run up to him, repeatedly.