*This can be done with a minimum of 2 people.*
This may not be the best method to do this achievement, but it is certainly worth mentioning.
First off, it is unnecessary to be in a Posse together. This will help with the chance to find an empty lobby.
1. Hit the Back(<-) button and select Legends & Killers Gang Matches
2*. Keep doing this with your friend until you or him get an empty lobby, and whoever gets it first needs to invite the other.
3. Immediately start a game by having both players select Ready.
4. Finish the shootout in the beginning (This cannot be skipped).
5. Have the person who is not getting the achievement leave to Free Roam.
6. The match will automatically be won. But you need to immediately open the Xbox guide and invite your friend back into the game while the post-game results are still being shown.
7. Repeat steps 3 through 6 until the achievement pops.
This method also works for the following achievement:
The Go Team! achievement in Red Dead Redemption worth 65 pointsBe on the winning team for four consecutive victories in any team based game in public matches.
But it is necessary for you to select Legends & Killers Free For All
*Part of the reason you do not want to be in a posse together when you start this is in the case that you do end up with randoms joining you for "The Quick and Everyone Else", the game should always put you on separate teams, which you can use to your advantange. Friendly fire is on...
*Also if you have 3 people working on this at the same time, you can kick anyone who attempts to join in the game.