Red Dead Redemption

Join a public free roam and then invite a friend. Posse up and then create a free for all match.

Now, when the match starts, have your partner leave, which automatically nets you a win.

Now when you go back the voting lobby, invite your friend to the game again and when the next match begins, do the same.
After your partner leaves the match for the 3rd time, you will get the achievement.

PS: make sure you kick whoever joins the match because for a small window of time you and your partner will have the majority guaranteeing the unwanted player is kicked.
This shouldn't really be a problem since the method takes mins and if you are in a fairly empty free roam, the chances of people joining your game are pretty slim.

Credit goes to "Faris Hilton" for the original solution for the "Go Team!" achievement, but it also works for this :)

The Quick and Everyone Else...sounds like it is a daunting task. It is actually very simple. What you will need for this is one boosting partner. I chose one of my buddies on Xbox Live. This will take you about 30 minutes to get at the most plus another 30 or so minutes for your boosting partner to get.

What you will need to do is start up a Public Free Roam session. While in the session invite your boosting partner and meet up in the town of your choice that has a FFA marker. I like the town of Armadillo because it is small and has visible markers. Once both of you are in Armadillo make your way to the FFA marker. It helps if both of you are standing by each other next to the marker. Start a FFA match and have your friend join it. Once you see both of your names pop up on the upper left hand corner start the match by holding down the Y button.

Now most of the time since you were standing right by each other at the FFA marker you will be thrown into your own FFA match. Wait for your boosting partner to show up then vote on your favorite gametype. Once the gametype is chosen immediately start the match so it is just the two of you. I prefer playing goldrush because I can capture the goldbags quick, and you only need to capture 10 of them in 10 minutes. I can usually get all 10 bags before the 10 minute time limit making this gametype go by much faster.

After the match vote to play it again. Once gametype is voted on again start the match as quick as you can. Other players can join in at any given time so be sure to give them the boot! This is known in this game as Vote to Kick!

Doing this method Quick is the way to go, and well Everyone Else...Well they will be left in the dust!

Just got done doing this with five people. You start/join a public free roam, invite your party, then posse up. Walk up to a match marker (I recommend Armadillo) and start the match. We did this in shootout. Pick a person to get the achievement, everyone else just spawn and run towards the middle of town. The one person gets 50 kills, repeat three times, switch to another person. If you get any randoms just kick them quickly, they shouldn't be a problem.

With 5 people and this method you can finish off one person in about 20 minutes. Might be faster than the Gold Rush method mentioned above. A good shooter will be able to get 50 kills right around the half way mark of the match. We got 5 people done in just under 2 hours.

The Le Matt revolver seemed to work the quickest, it is powerful and holds 9 shots. Might depend on your play style though.

*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:

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.