You will fight the Meth Tweekers during Tweek's recruitment mission. You'll be asked to retrieve a package from Kenny's garage which is where the Meth Tweekers are located. Before doing this, you must get the bald cap and the Evil Cartman goatee.
Bald Cap- Go to the police station and go upstairs. Go to the left and enter the locker room through the vents. There is a key in here you need. Next, go to the first floor and enter the cells on the left. Use the key to open the cell and get the Bald Cap in the bag.
Evil Cartman Goatee- Located in the drawyer as soon as you enter Cartman's house.
Once you have them both, equip them.
Now progress through Tweek's recruitment mission and go in Kenny's Garage and fight the Meth Tweekers. Afer the fight, the achievement will unlock.