Getting this bonus requires a bit of luck to get. For able to get "Massacre" bonus you have to kill 30 enemies in less than 5 seconds between each kill. This particular bonus will pop up after "Killstreak" bonus. This might sound scary but believe me, enemies will attack towards you in great numbers during first missions.
My own advice; once you encounter "horde" type of enemies, let em chase you while you are searching new enemies. This will stack the enemies around you for easily to kill them with AOE attacks. You will get this achievement before you even know it or just accidentally playing the game further on.