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The Big Beatdown is a side mission in South Park: The Fractured But Whole.
Mrs. Cartman contacts The New Kid and tells that her adult tutoring business has attracted the attention of a competitor. She requests The New Kid to "be a good daddy" and drop by her house for assistance.
As you walk on the streets after you completed Vigilante Marketing (usually past Token's house), Mrs. Cartman will call you and inform you of her situation of having attracted attentions from a "Competitor". After getting the video call from Mrs. Cartman, simply head to Cartman's House. You will see a pimped out car parked in front of the house. Enter the house to find Mrs. Cartman being confronted by a pimp named Bishop, accompanied with two ladies. You will then be engaged into combat with the pimp and the two ladies.
Battle: Slap down the pimp!
In this battle, all of your abilities will be replaced with a single attack, labelled as the Pimp Slap (your inventory slot still remains available though. Backstory You can also still use their respective powers). Use this attack against the ladies, who will be affected by the Charmed status. After using Pimp Slap on the ladies, they will proceed to attack the pimp.
However, the pimp also has the same attack as The New Kid. When he attacks any of the Charmed ladies, the Charmed status will be eliminated. Simply just slap the ladies back to get them Charmed again. You also can personally slap the pimp, however it does not do as much damage as the ladies do when they get Charmed by your attack.
Once the pimp is defeated, you will be rewarded with the Goblet of the Supreme Crunk artifact and the Pimp Strength DNA slot. Proceed to take selfies with the pimp's ladies (named Cristal and Shawna respectively), the mission will then be complete.