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