To get this achievement, you are required to stick the opposing flag carrier with a plasma grenade and having it detonate while they are still alive AND holding onto the flag. This achievement is not too hard to do legit but can also be done easily by boosting. If going for it legit, I recommend BTB 2 Flag on Ridgeline. If boosting, the Squad playlist is better because it only requires 8 players not 16.
Boosting: The best method would be having each team bringing the enemy flag back to their base but not capturing it. That will allow each team to spawn close enough and grab the enemy flag again so it will not reset itself automatically.
Legit: The strategy I used was I first grabbed plasma grenades and then hid in the back of my base until our flag was taken. On Ridgeline, the plasma grenades spawn down the hill to the left of your base next to the ghost. Some people may prefer the camp above the flag but the problem I had doing that was that one or two enemies would rush up top via the hill and kill me.
And finally good luck with this, either going for it legit or breezing through it by boosting!