Warface

The method I used to unlock this is pretty simple. First you need to unlock the Initiation Mission: Path B in Co-Op, after you get off the helicopter run to the stairs in front of you. Select the grenade, left on the D-Pad and when you're almost reaching the door on the top of the stairs, continuously press RB button, if you pay close attention three guys will come and more from behind those, just rush until you are close to them and when you are they will melee you to death. Before you die the grenade will kill them and you will get the achievement.

I think the easiest way to pop this achievement is, At the start of a co-op mission, Run out of the chopper and forward through the map just a little bit till enemies start spawning then simply run back inside the chopper and let them gather outside the windows, Now cook a grenade and throw it out, Dont worry about killing yourself, The achievement will still pop!