Express Checkout 25G
Kill 4 players within 3 seconds in the Imperial Palace

---

If you want to go for this legitimately, the easiest way to pull this off is to unlock the Weapon Double Dealer Burst perk which is available at level 48, when this is triggered at level 3 all the enemies grenades will instantly drop and explode. When you trigger this you need to be in the line of sight of at least an enemy player and be ready to kill them, hopefully causing a chain reaction to the other players on the team and blasting them all.

Although that last method still seems quite challenging, this is a challenging achievement therefore you can always boost it by getting together 8 players again and start up a public Team Deathmatch game. Then gather 4 enemy players in a small area like inside the empty swimming pool and either blast them with an RPG or chuck a grenade.