Not A Rabbit is an achievement/trophy for Metro: Last Light awarded to the player if they can finish the level Ashes without taking any damage from the watchmen. Presumably taking damage from falling will also negate it, but there are very few places this can happen. It is best to complete this on a high difficulty level, as the watchmen will take a lot less damage to kill. Although Artyom will also take more damage, you should kill them before they can reach him. A machine gun, such as the Kalash or RPK-74, is preferred.
Should the player get hit during the first encounter with the watchmen, you can restart from last checkpoint and try again without voiding the achievement/trophy. A video guide is featured below.