If you're fairly lucky and most other players aren't aiming at you, you should be able to kill three other people while dodging out the way. You may need to play several standoffs/showdowns to do so, but it depends how quick you are at drawing. I recommend you play a standard playlist and not the advanced as then you can take advantage of the lock on feature with the draw. To snap between players, aim towards someone and then quickly tap the to snap onto the player. Keep doing this until you have killed three other players.
Alternatively you can find three other people to join you in a Free For All match, then have them stand there while you take it in turns to kill each other for the achievement requirements.
See "Call it a Comeback!" for boosting information.