In Green Run, pacify at least 10 zombies with 1 EMP

Start a solo match on TranZit and ride the bus until you reach the next bus stop. You will then be at the "Diner". Look to the sky and search for a blue beam of light. This blue beam shows the location of the mystery box. The mystery box always spawns at the Diner. EMP Grenades can only be obtained from the mystery box, so be prepared to spent some points on it. Each try costs 950 points! Just keep killing zombies for a few rounds and use the mystery box whenever you can. Once you have the EMP grenades, round up a group of zombies. Throw the grenade when 10 of them are standing right next to each other. In case the mystery box disappears, you can either search for its new spawn location (watch the blue beam) or restart the game