There is a trick you can do which makes this achievement easier to obtain. You only need a 2nd player to help you do it. I used a dummy since I had another copy of the game.
Whoever is the host needs to set up a Ranked Capture The Flag Match...Map: Rubble
Time Limit: 30 Minutes
Capture Limit: 1
Join Grace Period: 0
Respawn Time Limit: 1 Second
Everything else can be kept the same. By changing these settings, whoevers the player getting killed will have to press the
button to respawn. This is a great advantage.
When the match starts have the person doing the killing to to the bulding on either the left or the right side of the map. There are 2 ways you can access the building, which are either by the slope, or by jumping on the 2 wooden planks. Inside the broken part of the building is one of these Trip Mines. Simply get the person who wants the achievement to pick-up one of these and then throw them on the right side of the wall.
Meanwhile the other player who is getting killed just needs to head to the same spot and just walk into the Trip Mine. Rinse and repeat until the person who's going for the achievements gets 50 kills. This is quite a long boosting method, since it'll take around 15 - 20 minutes.
Now the trick for the player using the trip mines is that originally you would have to walk backward and forward to grab the trip mine, when it appears. It turns out that even if you press
to jump and land on the trip mine when it respawns, you'll pick it up.
Once the player has got 50 kills, you can either swap over and let the other person go for 50 kills, move onto another weapon for the player who is doing the kills, or you can just capture a flag and end the match.
If the other player wants the achievement as well in the same match, the best method is to swap roles and have the player who was going for kills to get killed. Then the player which was getting killed heads over and perform the same boosting method.
You need 50 kills to get the achievement, however the achievement will not unlock until you have completed the match. If you lose connection whilst going for the achievement, none of your progress would have been stored.