Thanos was no match for Ka-Zar, as the powerless Marvel hero defeated him in such a humiliating fashion, the story was later retconned. In Ka-Zar #11, the Mad Titan faced off against the protector of the Savage Land, where the two sparred over a bubbling volcano. Thanos seemed certain he was going to win the battle, but ultimately was kicked into the lava by Ka-Zar and had his armor burned off his body.
Ka-Zar might not be a household name to non-comic book readers, but he's been an important part of Marvel history for more than 50 years. Created by Jack Kirby and Stan Lee, Ka-Zar first debuted in X-Men #11 as the "Lord of the Jungle." He's essentially the Tarzan of the Marvel Universe. Ka-Zar lives in the Savage Land, a tropical land hidden in Antarctica where prehistoric life, including dinosaurs, still exists. He originally tangoed with the X-Men in some of their earliest adventures. Ka-Zar and the Savage Land still appear in Marvel Comics to this day.
In Ka-Zar #11 (1997) by Mark Waid, Andy Kubert, Joe Rosas, Digital Chameleon, Jesse Delperdang, and Todd Klein, Ka-Zar's brother Parnival worked with Thanos to use the same alien Terraformer that turned the Savage Land into a prehistoric area on New York. The plan was a success, as the concrete jungle turned into a literal jungle. Thanos didn't stop there, turning the Kree, Inhuman, and Shi'ar home planets into jungles. Ka-Zar and Thanos battle across space, taking portals to different terraformed planets. The fight eventually leads them to the Savage Land, where they fought up a volcano. Thanos has Ka-Zar in his grasp when the hero uses his strength to break free and kick the Mad Titan into the lava below.
The kick doesn't kill Thanos, as he emerges naked from the lava afterward. Thanos sets the Savage Land on fire, which leads SHIELD agents to detonate a bomb on the terraforming machine. The blast returns all to normal as Thanos is transported away. Funny enough, the story would be later retconned so that the version of Thanos Ka-Zar faced was actually one of his Thanosi clones. According to CBR.com, the retconning was done by Jim Starlin to make the Mad Titan seem like he was unrivaled in battle - as almost five years' worth of Thanos stories were changed.
The retconning might have made it so the Thanos that Ka-Zar faced wasn't actually Thanos (even if it was meant to be at the time and was never alluded to otherwise in the story), but the defeat is still incredibly humiliating. A hero with no superhuman abilities defeated Thanos and stopped the universe from being terraformed for good. Move over Squirrel Girl, Ka-Zar is probably the oddest choice to take on Thanos one-on-one and win in embarrassing fashion.