This may seem difficult, but there are ways around obtaining this quite easily. You must be on the Creature team in order to do this, but take note that only the player who finishes off the Rookie will unlock the achievement.
This is best done in a Private Match and if you spawn as the Creature first. Have your Human partner(s) choose Helena, since she has grenades in her loadout. In the match, the Humans should throw grenades at the Rookie in order to weaken them. Now as a zombie/B.O.W., run to the Rookie and finish him off. When the round ends, the achievement is yours.