*NOTE:The first solution I have posted, as pointed out by Demonhunter 7, is now bannable by Bungie. (wasn't when i posted this, but now is none the less). I would strongly suggest using the second part solution i have posted unless you don't care about getting a credit ban. (if you don't want to get a credit ban for using the first method, get the achievement and then just make sure to distribute kills amongst your guest and yourself. What the "bungie banhammer" is looking for when banning is full parties that have only one player with all the kills on each or one of the teams.when it recognizes that only 1 or 2 players have all the kills it will hand out the bans. Bans will not allow you to earn credits in matchmaking for a week. you will still be able to play, but won't earn any credits for doing so.
The Best way to get this achievement is boosting. You can have up to 3 guests in multiplayer, so if you and a friend each have four controllers, you only need 2 people. Just have your friend place his 2 guests against a wall and grab a shotgun or sword (or any1 hit kill weapon, these are the 2 easiest to use though) and stand behind you. once everyone is in position, take out the 2 people you plan on killing's shields and throw a plasma grenade above their heads and have your friend immediately kill you. if you did it correctly you should die before the grenade goes off and get the achievement!
If you don't want to boost, this will take time and a little bit of luck (or lack if look depending on how you want to look at it). The best mode to do it in is either Invasion or Big Team Battle. The reason for this is that there is lots of vehicle play in both.
Your best bet is to get invasion on Spire. in the round when you are on the Elites team, Try letting the Spartan get to the third phase. During this phase the Spartans get a Warthog (also available during phase 2) and a Falcon. Both of these vehicles carry up to 3 people.
when you spawn in phase 3, you should spawn behind some cover that looks like the purple thing in this pic http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=10783...
(note: you can chose to spawn here, if you don'd see this landmark press up or down on the d-pad and it will move your spawn location). Once you spawn here, head to the right and continue along the path http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=10783...
(pic for reference) until you get to the plasma launcher, which is located behind the rocks shown in the previous pic.
Once you get the plasma launcher, move to an area that you can jump off the edge and wait for the other team to grab the Falcon and get in range. use the right thumbstick to zoom in and wait for the reticle to turn red. Fully charge up the launcher and fire all four rounds at the Falcon. Once you see the rounds are tracking, jump off the edge and enjoy your achievement. hope this helps :) Good luck!