I did this in less than thirty seconds last night,
Load up any anniversary free for all gametype in matchmaking, with you and a second controller or boosting partner.
Have your second controller/boosting partner throw a sticky grenade at your primary account, make sure the two players are stood right next to each other.
When the plasma grenade detonates it should kill both players netting you the achievement.
If you are boosting with a partner, rinse and repeat swaping roles.
Hope it helps