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 SecondEverything 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 run over to the opponents base. Now originally the opponent will be standing in front of the flag, kill them here. Now the trick is that is the player going for the achievement stands in front door's archway, which contains the flag inside the building, the opponent can only spawn in one area on the map. The spawn area is on the right side nearby a dumpster, and the garage door. You can easily see the opponent.
The map only has 3 - 4 spawn points, and because the killer is standing in an area which is close to the spawn points, the game registers that it can only spawn the player at the garage door, dumpster area.
The melee attack is simple, in which you just run to your opponent and simply perform a melee kill whilst on foot.
When the player who is dying gets killed, they must "NOT" press the
Button to respawn until the player who's killed them heads to the area where they need to stand, which is in-between the archways that contain the flag that's inside the next room.
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 to their opponents base and perform the same boosting method, since both bases contain the same dumpster & garage location.
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.