This achievement requires you do get 250 killjoy medals in matchmade games. Killjoy medals are obtained when you end an opponent's killing spree (5 kills or more). This achievement used to be quite simple with a glitch explained in Spoof Ex's guide, but unfortunately, the April 21st update fixed this glitch.
My guide will center around two people boosting with four controllers. Boosting with eight people is just too inefficient. Gamestop offers a weeklong return plan for used items. This allows you to purchase three other controllers, finish multiplayer achievements, and return them for a full refund.
I would highly recommend completing every other matchmade achievement before focusing all your efforts on this one achievement. However, while getting all the other matchmade achievements, get killjoys whenever possible. For example, when you and your partner are focusing on Domination (Earn a total of 100 Legendary medals in matchmade games), get a killjoy for every two exterminations your partner gets. Basically, just try to kill your partner when he's up to a killing spree.
Both you and your partner are going to be searching in the Team Hardcore playlist. Start searching at the exact same time. If the game says "Players Found" and "Connecting Session" at the same time, you know you've been matched up with each other. Back out immediately if not.
The map you will want is Midship on CTF or Assault. There are only five maps in the playlist, so it shouldn't be too hard to get it.
At the one minute mark, have one dummy take the flag/bomb and stand up at the top at sword. Standing up at sword allows for less confusion. If someone is holding on to the objective, the game will never end.
In a condensed form, you and your partner will be running around with a guest, getting a killing spree, and dying by the hands of your main account. It's inefficient to earn killing sprees with your main account. Other than that, its mostly tips to save time. Make sure to have the other team hold the flag/bomb when you're getting too far ahead. This will balance things out.
Communication is the most important part of getting killing sprees. When a guest spawns in, make sure to call out the location. Feel free to make your own callouts. The callouts I used were:
Top (initial spawn - top part of base)
Bottom (bottom part of base)
Pink (pink part of the map off to the side)
Pink Top (above Pink)
Silver (opposite from pink, domish, and slightly silver)
Silver Top (above Silver)
Outside (outside part of base, close to middle)
When a guest gets a killing spree, put them down in the middle by shotgun or in front of your partner's main account, and call it out so your partner can get a killjoy.
Other than that, its mostly practice. Expect the first few to take longer than others, but you will develop a rhythm. If any of this didn't make sense, please leave a comment. I try to be fast to reply. Credit to Mutantkilla for being my partner for this achievement and helping to create this method. Both Mutantkilla and I managed to get this done in around 3.5 hours of boosting, granted we both had about 30% of it done from the other achievements.