Day Walker is a missable achievement in South Park: The Stick of Truth. It is worth 15 gamerscore and is awarded for defeating the Boss Hall Monitor while wearing freckles. You will encounter this boos during the quest “Call to Banners” while rescuing Craig from the school.
The easiest way to obtain freckles is to simply choose them at the character creation screen. However, if you already have a character without freckles, don’t despair! There is another way. While completing the quest, you will encounter a single Hall Monitor before you reach the boss. This enemy will always drop the freckles you need to complete the achievement. Just make sure you equip them before you initiate the boss encounter and the achievement shall be yours!
This short video shows which enemy you can obtain the freckles from:
Thanks to Enonymous1 Gaming for the video.