This Trophy can be achieved in either Campaign or Zombies and the later is the easiest way of achieving it, load up Kino Der Toten and purchase the M14 of the wall, this eliminates the random element of needing a weapon powerful enough, 6 Zombies spawn in the first round, wait for them to all group up and then when you have distance aim for the heads and pull the trigger with that at least 3 should die and the Trophy is yours.