"In TranZit, kill one of the denizens of the forest while it is latched onto you."
To get this achievement you need to be in Zombies mode, playing a TranZit game. Open the main doors and then run out into the wilderness, into the smog until you see a skull and crossbones sign out along the road. It is around here that the 'denizens of the forest' will emerge.
Once one latches onto you, use the right stick to melee it. It will take about 5 swipes to kill it and get the achievement.
During the courtyard section with the bell tower, you may find that you are unable to continue, with the game seemingly glitched. It is in fact waiting for you to kill everyone in the area - search all the corpses. You will likely find one that is still alive. Finishing him off will allow you to proceed. That, or kill yourself and restart the level...