1) Begin the co-op Special Ops mission Sniper Fi on recruit.
2) Wait until the predator missile is available to take.
3) Take the predator missile.
4) Wait for the round to begin.
5) When the round begins use the predator missile.
6) Fire it when most of the enemies are together.
7) As you line up the predator for impact, have your friend knife you to put you in last stand.
8) The predator missile will continue down to hit the enemies.
9) If the predator missile stayed in line and killed four enemies the achievement will immediately pop up.
10) If you missed, just try again next round.

Here is a video tutorial: