Campaign: Dark Carnival
Level 2: The Fairground
If you have captions on you will see the shooting gallery making noises. Run straight from the saferoom across the gallery, you need to score over 750 points by shooting the various targets. Just to the right of the gallery you will see how many points you will earn for each target. Watch out for the peanut as he will lower your score by 100 points.
Once you win, a box to the right of the gallery will open up and reward you with Gnome Chompski. Allocate whoever will carry the gnome and make your way to the end of the campaign. Do not drop the gnome. Some points on the roller coaster will cause you to lose him and you will have to either die or start over.
Once you get to the Finale do not drop the gnome. Hold onto him for dear life until the credits roll. If you drop him before the campaign ends you will not earn the achievement.