Guncraft Blocked and Loaded

A couple of things to note for this achievement are that you need to be around rank 23 to host a racing lobby. The other players can then be invited to the game regardless of their rank.

Also you can use a second controller, effectively meaning you only need 8 people. From the main menu just pick the xbox live option then press A on the second controller and sign in a guest.

Make sure the score to win is set to 1 and use The Lava Pit map as it is very small and a race can be won in around 25 seconds. Just pass through the purple checkpoint markers and make sure to avoid the powerups which are multi-coloured blocks with question marks on them.
Once the winner has crossed the finish line there will be a 1 minute timer counting down to end the match so the other people can just wait at the start.