Very early in the game you are asked to recruit Tweek. After speaking with him he asks you to get something for him in Kenny's garage. You need to equip a bald cap and the evil cartman goatee before the encounter.
Bald cap location- Police Station 2nd floor locker room (accessed through vents), obtain key then go to jail cells on the 1st floor inside a bag.
Evil cartman goatee location- Eric's house first drawer by entrance.
After the battle, achievement unlocked.
If you missed it, start a new game and it should take about 10 minutes.
*Again make sure you have both items Equipped and not just in your inventory!
For those of you that are visually inclined here is a video guide.
All credit and thanks goes to community member oG II GiiFT3D x