For this achievement and the dropshot one, for easier multikills, i suggest you to use the lancer as second weapon.
Play casual bot, pick up your boomshot/dropshot and use the lancer to shoot down one guy, but dont kill him, wait for his teammate to come revive him (they do it almost everytime if they are close and they rarely care about you while they are coming to rescue their mates) then just shoot with your heavy weapon when he get close enough and you get your double kill. I got the achievement in 2-3 games that way.