For this achievement, simply load up chapter one of the passing. There is a chance that a golf club will appear at the start location. If the golf club is not there, either call a vote to restart the campaign, or go to the main menu and restart the campaign manually. Eventually a golf club will appear where you spawn. Once you have it, simply aim for the zombies heads, swing, and pop them off. Once you do this 18 times, the achievement will unlock.
See the below video for extra help (Credit goes to Rooster Teeth)