During Psych Warfare matches after the other team attach the battry to your teams motivator and the countdown reaches zero, another countdown will begin. During this time you will not be able to shoot, though if you hold the fire trigger you will perform a slapping animation.
Kill the enemies with this slap 50 time and you will gain the achievement. It's very hard as opponents will be able to see you on their radar during this time and their gun fire pushes you back. It is best to ambush enemies as they chase your team mates.
You can also use the panic-slap melee attack at the end of a losing Fumigation match, but the window of time in which it is possible is very small, before you die from the poison/bats.