This achievement is obtained by pushing a Shadow / Zombie off of you while being grabbed. This can be the very first achievement earned in the game. When you start a new game walk about 3 rooms to the right and when you see a ladder STOP. A zombie should enter the room behind you, allow this zombie to grab you and spam B as fast as possible.
If you don't get it in the beginning you will most likely find yourself in a sticky situation later in the game. As said before, if a Shadow / Zombie grabs a hold of you quickly mash the B button to push them away. If you manage to get them off of you without losing a health bar the achievement should pop. (I'm starting to think health has nothing to do with the achievement as much as just kicking the zombie away.)