Halo 3

Easiest way to get this is to wait for a Free-For-All King of The Hill game that takes place on a level with a Rocket Launcher.

Get the Rocket Launcher, get a safe distance from the hill, wait until there are 2-3 people fighting each other and blast them. As most people gravitate around the Hill, this gametype is ideal for trying to get this achievement as everyone should frequently be in the same area. And if you are focusing on killing them, instead of getting inside the hill, you will have an advantage there.

Note: be prepared to lose a few games while you try for this. Remember, you should be focused only on getting this achievement, not on winning the match, and you should get it fairly easily.

Good luck!

This solution applies to most Free For All Achievements in ranked matches.

Important!!
Get into a boosting session with 6 other players within 10 ranks of each other to fill up a room. The rank of the highest player must be within 10 ranks of the lowest player for everyone to get into the same room.

Using the Language Glitch Below:

To do the language glitch, go onto your Xbox 360 Dashboard.
Go onto the "My Xbox" section, and go to the furthest right option titled "System Settings".
Select "Console Settings".
Select the third option down, titled "Language and Locale".
Select "Language".
Go to your desired language, and press A. Then press B until you're finally back on the Dashboard.
Load up Halo 3 on your XBOX, and memorize which option is Matchmaking; I believe its second from top option for Halo and third from the top for ODST.

When in the lobby (before searching), press X, and select the bottom option. This makes your search customized, and is basically narrowing your search so that you are only paired with other players who are using the same Language on their Xbox as you.
If everybody else has changed their Language to the same as yours, and searches at the same time as you, then you shall be paired relatively quickly. Then it's boosting time!

Now part 2:

If you have less than five players (Once in session go to the center of the map. Use the shotgun, sword or simply assassinate other players. Try to get to a smaller map and count two to three seconds between kills. Have the first guy run back to the center of the map hopefully within 14-16 seconds to get achievement to pop)

If there are 5 or more. Well simply kill everyone within three seconds of each other.

Hope this helps

Prob the toughest achievement of the original 1000GS..
Has to be done in a ranked Free-For-All playlist, either Lone Wolves or Mythic Brawl.
Mythic Brawl is a double exp weekend playlist with 8-12 (pretty sure that's it). If it comes up it might be good to try there as your rank might be lower and there are more people to kill.

For Lone Wolves, it's easiest in an objective match like King of the Hill or Oddball but Swords works well also.

Swords: This is how i got the achievement, using Melee (B or RB for bumper jumper) as opposed to right trigger as it gives you the quicker slash if you clash swords with someone. Also try to find grenades because if two players clash, their shields go down and you can get an easy double with a frag.

Oddball: an easy solution is to camp in a bubble or behind a shield door with the ball and melee. but if you trade beat downs with someone you ll be left with no shield. id recommend using the ball as bait in a room and try to pick up kills with a shotgun (boundless, guardian), sword (construct) or the rockets (foundry). grenades also work well as people are focused on getting ball time and are distracted.

King of the Hill: doesn't come up too often, but works as well. if on The Pit id recommend using the rockets or sniper over the mounted turret as you can generally get two-for-ones if you aim your shot better. Good position to fire from is either from the sniper tower or the needler spawn as people are less likely to spawn behind you.
On Boundless, grab the beam rifle and the shotgun and id just spam grenades and use the sniper or if you want to try your luck with a bubble shield in the centre of a hill and camp with the shotgun.

If you're going to try on the Mythic Brawl DXP, there are some really good gametypes also:

Brawl Rockets: infinite rocket ammo with 12 people on the map. Try to aim for two at once. also because of all the rockets flying around, people are constantly on low shields so melees and grenades work too.

Brawl Swords: see above.

Brawl Fiesta: I believe this is the easiest gametype. Run around with shotgun/sword and rockets and you'll get it easy. the Battle Rifle works well to as most use rockets so you can easily clean up kills with headshots.

I hope this guide helps people.. Best of luck!!!

Tip for Boosters!

this is not a solution for the achievement, but for what comes after... (don't hate me for it, it's just to help)

If you managed to get it with the help of the Language-Trick, you'll probably have noticed that the name and description of the achievement is not English (or whatever).
To solve this you need to do the following for pre-NXE:
1. Go to the Xbox-dashboard
2. Navigate to the "Marketplace" blade
3. Press "X" to Sign Out, confirm it with Yes
4. Go to "Recover Account"
5. Enter your Gamertag and Continue
6. Put in your NET password and ID and continue
7. Wait for it to recover and check your English Achievements

for NXE:
1. Go to the Xbox-Dashboard
2. Press the Xbox-guide button
3. Press "X" to Sign Out, confirm with "Yes, sign out"
4. Click the "Recover Gamertag" button, and again.
5. Confirm to exit your current session with "Yes"
6. Enter your Gamertag and Continue
7. Enter your Live ID and your password and Sign In
8. Accept the Xbox LIVE Terms of Use and Privacy Statement
9. Wait until your profile is downloaded and check your English Achievements

The solution is not invented by me, although I wrote down the NXE version with the help of the following webpage: http://www.myxboxspot.com/forums/halo-3-glitches/1387-how-ch...
The pre-NXE solution is a retyped and shortened version of the idea that can be found on the same webpage, I have copied it because I wanted to spread the word about this solution.

I got this achievement in Lone Wolves during an Oddball match on Guardian. It was purely accidental and I'm sure if anyone tried they'd do it even easier. I think I used an assult rifle to kill 3 of the other players scrambling for the ball and then killed the ball carrier. I also got a triple kill whilst in posession of the ball while in a bubble shield and so that seems a possible solution.

This achievement is highly down to luck and circumstances. Be prepared to lose some matches and go down in rank whilst pursuing this achievement.

GOt this one in a double xp thing a while back with a lot of players. Got a rocket saw a whole bunch of people having a brawl and killed the all. It will take you a while unless you are focused.

I made a video of me unlocking Overkill, Triple Kill, and Steppin' Razor to show you how I got mine. Just use a sword in a close quarter map and kill everything that moves and make sure you swing your sword first. If you don't want to do it this way or already have the other 2 achievements you can just grab a rocket launcher and try to get lucky.