This achievement is buggy for a lot of people, but here are a few tips on how to achieve it. First, and foremost, in order to obtain the achievement, you must delete your save and start from scratch before every attempt at going for this achievement. For some reason, if you don't do this step, even if you fulfill the other requirements, you probably will not get the achievement. You can always copy the save to a memory card though.
You should also try to gather as many people as you can in a party to help you boost this achievement. While you may not get a game of just you and your friends, the more people you have trying for this, the better your chances of getting more people in the same game. It is a good idea for everyone to prefer each other so you can find each other easier. Everyone should go to the cops and robbers game mode, and search at the same time for a game. Once someone is a host with their own room, have them put the level on "Airport" and wait. "Airport" seems to be an easy level to meet up with people at, however, you don't have to go with this one. Everyone else should keep searching until they get into that room. As soon as there are 6 players in the room, regardless of if they are all in your party, the host should start the match. The reason for this being, if you have 6 people, and you have even 1 or 2 others from your party in the game as well, then your chance of getting the achievement increases greatly.
Decide on who is going for the achievement, and whoever is on the opposite team, should go up to that person and let them get kills. If the person getting kills goes to the other player's spawn point, then it becomes a lot easier to rack up kills faster. If you have randoms in the game, keep an eye on where they are, so they don't start to steal kills, or kill the person going for the achievement. Also, don't worry at all about the loot or escaping the helicopter, the achievement is only for getting kills.
Between rounds, pay attention to the amount of kills in the columns, and make sure they total up right. For some reason, if they don't, then there is a chance that you may not get the achievement. Also, at the end of the game, it seems to help if you have at least 10 more kills than the person in 2nd place as far as kills go. While this isn't definite, it has said to help in terms of getting the achievement. Another way that the achievement may not be achieved is if someone quits out of the game, so when playing, keep that in mind too.
Keep in mind that this achievement is glitchy, so even if you do everything in this guide, there is always a chance that you won't get the achievement. I met the requirements two times and didn't get the achievement, however, I was able to get it on my 3rd try.