The easiest way to do this is to do would be, what I did, was to have multiple controllers (minimum 2), and play by yourself or with a friend.
Since I did not have any of my friends over at the time, I had the second controller inactive, while I was using my main controller.
From the main menu, I went to Local Game, and added the second controller in by turning it on and pressing
.
By default, you should spawn with the Blunderbuss, if not, just use the Left & Right Bumpers to cycle through the Weapons.
Pressed play on the main controller.
Once you reach 5 Kills, the game will end and the Achievement will be unlocked.
I will try to get a Video Solution very soon of my playing the game within the next couple of hours, I just couldn't record it at the time of playing and post it below, though since I have already unlocked the Achievement, it won't unlock for me, but it will for you.EDIT: Here is a video below of me playing against myself (separate controller) on one of the maps: