On South Park, Eric Cartman is a legendary jerk. Outside of Butters, no one is more aware of that fact than Wendy Testaburger. Stan's on-again, off-again girlfriend has had more than a few run-ins with the scourge of South Park Elementary over the years. But she was also the one to finally take Cartman down a peg with a good ol' fashioned butt whipping.
Calling Cartman offensive doesn't do the depths of his bigotry justice. It doesn't matter what race, religion, gender or sexual orientation one identifies with, Cartman has a slur and an offensive stereotype lined up for them. But that's what makes Cartman who he is, and any attempt to change him has backfired spectacularly.
Stan, Kyle and Kenny find him just barely tolerable, and they are supposedly his best friends. However, when it comes to women, Cartman definitely has a feeling of superiority. His misogyny has been directed at virtually every woman he knows, particularly his own mother. The girls in his class almost all universally hate him. But things could only go so far until somebody pushed back, and that someone was Wendy Testaburger.
In Season 12, Episode 9, Wendy was trying to give a presentation on breast cancer awareness. This made her an obvious target for Cartman, who brutally heckled and jeered. Wendy finally reached her breaking point and called Cartman out, challenging him to a fight. Being a coward, Cartman did everything possible to avoid fighting Wendy. When none of his begging worked, Cartman got Wendy's parents involved, and they told her to stop bullying Eric. This gambit briefly paid off, and Cartman went right back to tormenting Wendy. Defeated, she resigned herself to a life of dealing with Cartman's crap.
Then, Principal Victoria stepped in. A breast cancer survivor herself, Victoria was unhappy to find out that Wendy was backing down. She had her own run-ins with Cartman over the years and took a much different stance than most educators would have. She encouraged Wendy to go through with the fight, comparing Eric to a nasty little lump that needed to be removed.
The end result was a world-class beating that left Cartman lying in the mud, bloody and spent. Wendy walked away, leaving all the kids at school in awe. Except for Butters, who found Cartman getting pounded into oblivion hilarious and deeply enjoyable. It's honestly hard to blame him, given how cruel Cartman had been to Butters in the past.
Cartman should have taken Wendy more seriously. After the Miss Ellen incident in Season 1, Episode 11, Cartman should have known Wendy was capable of some pretty brutal actions. What Wendy realized was sneaky plans and shenanigans would not be effective. Physical violence is the only thing Cartman understands, and she was happy to oblige, even if it didn't change Cartman in the least.