Seeing as the Coalition took over the rights for the Servers in this game, some things changed quite drastically.
First, only 2 online versus achievements are doable in private matches.
Second, you can either play Social or Ranked games, both work. Although i need to mention that you can choose the gametype in social, as for ranked you are bound to use whichever gametype the coalition put into the rotation (most of the time 1 or 2 different gamemodes).
Third, you need more than 2 players! This is crucial, since ALL the other solutions tell you you can do this with as few as 2 players with 2 second controllers. This is partly true sadly.
For the gametype gnasher execution 2v2 you need in total 4 players, either 4 real players or 2 players with guest accounts.
As for the other gametypes, a minimum of 8 is required to even match up. So either 8 real players or 4 with guest accounts.
For this achievement i recommend the 2v2 gnasher execution. A match can last up to 7-10 minutes as long as only one player is killing. Make sure to rubberband the guest controller and check every now and then if he is still moving, disconnects happen in a matter of minutes due to inactivity!
If you happen to downvote, please state at least why, i will take this solution down if it takes too many downvotes. So, stay fair and let everyone on TA benefit from this!