BEFORE GOING FOR THIS IT IS RECOMMENDED THAT YOU GO TO THE MAIN MENU AND MAKE A COPY OF YOUR SAVE SO YOU CAN DO BOTH OPTIONAL PATHS WITHOUT RESTARTING THE ENTIRE EPISODE.
This achievement is gained during Act 2 after choosing to go Hunting
. The Hunting section has two contradicting achievements, so make sure you back out after completing this to get the other. Once you gain control of Clementine, speak to Louis who will get you to swing a bat at a hanging walker. You will need to aim at their head, and press RT when the cursor turns red to hit them. Hit the hanging zombie 3 times in order to unlock this, The following missable achievement can be seen in the video below at 6:55
If you miss a swing, you may have to quickly back out to the main menu to restart this section, or revisiting a manual save.