Elite Killing Team Achievement
Elite Killing Team15
Dual Executed a helmeted enemy
This achievement must be done in COOP with a human partner. Have one of the players fill up their Mark & Execute gauge. Now mark a helmeted enemy by tapping over them. When both players are near the marked enemy, there will be a "Hold " prompt on the screen which invites the second player to join in on the execution. Once the second player accepts, you will dual execute the hostile and unlock the achievement.
You CANNOT do this on Perfectionist, as there is no Mark & Execute ability.