Equip the 'Explosive Burst' to any of your loadouts. Hop into a Gang Wars game and proceed to get a couple kills until your Burst meter reaches Level 3. Melee kills will fill up the meter faster than regular kills.
This Burst essentially drops a live grenade at the feet of all visible enemies on the map. Your best chance to get this legitimately would be to wait until there is a round of 'Turf Grab' , as the chances of the enemy team being bunched up are high. Once you see at least 3 players on the enemy team, activate your Burst and hopefully the enemies die from the explosion.
If boosting, simply follow the same steps to get your Burst meter to Level 3, then have your partners stand in a circle right in front of you while you activate the Burst. They will all drop a grenade and once the explosion kills them, the achievement is yours.