The Big Beatdown is a side mission in the video game, "South Park: The Fractured But Whole".
Plot details follow.
Head to Cartman's House to find Keshawn and his hoes threatening Ms. Cartman. Soon, a battle will ensue.
Simply Pimp Slap the girl in front of you. Then, she will turn around and kill the pimp. As a reward, you will receive the Goblet of Supreme Crunk artifact and the Pimp Strength DNA.
Take a selfie with each girl to complete the side quest.