The easiest way to get this is to first disable the alarms, this way, even if spotted, the alarm can't be used and you can freely slaughter your enemies to your hearts content.
Or, for a more fun approach, find a camp that has an animal caged, preferably a tiger or leopard, and free it. Most of the time, the tiger/leopard will have no issues killing all the opposing faction. You'll get a ghost and no alarms bonus as well (assuming you weren't spotted).