Face Hoff is a missable achievement in South Park: The Stick of Truth worth 15 GamerScore. It is awarded for defeating Princess Kenny while looking like David Hasselhoff.
Before you venture into Clyde’s Fortress during the Beat Up Clyde quest, head over to Tom’s Rhinoplasty. Speak to Sharon (Stan’s mother) and buy The HasselHoff for $175.
If you decide you don’t want to spend $175 for the nose job, you can create a save, pay the $175, continue on to defeat Princess Kenny before reloading your original save.
Note that you should aim to complete this achievement along with the Two Girls, One Stick and Outpatient achievements.
Here is a video explaining how to obtain the Face Hoff achievement.
Thanks to titansfreak28 for the video. Enjoy!