While he may be a literal god in the Marvel Universe, Thor does have limits, but the most R-rated fight in his long history revealed that a surprising MCU ally is capable of expanding them to shocking new levels. Thor's epic strength upgrade took place against the Viking horde of Harald Jaekelsson - a millennium-old zombie warlord who initially humiliated the God of Thunder with his strength.
Cursed to sail for a thousand years by a misguided mystic, Jaekelsson spent the years growing in power until finally reaching America, leading his men on a blood-soaked raid of New York City. This all happens in Thor: Vikings, which was published under the Marvel MAX imprint - a line which targeted adult readers with excessive violence and unfiltered language. This lack of restrictions was essential for Thor's battle with Jaekelsson, as the viking hurt Marvel's Odinsson in ways few have managed before or since.
In his first clash with the Viking, Thor attacks with all his might, landing a thunderous blow with Mjolnir. Jaekelsson is so powerful, the blow doesn't even move him, but it does shatter both Thor's wrists in a gory show of force. After a shocking beating marked with similarly graphic injuries, Jaekelsson hurls Thor into the river, where he's thankfully discovered by Doctor Strange. In the 2003 series - from Garth Ennis and Glenn Fabry - the Sorcerer Supreme summons a group of the original mystic's descendants, using their connection to the spell to empower them and Thor for a rematch with the Vikings.
While Doctor Strange is powerful, few would guess he's capable of upgrading the abilities of a literal god. However, Strange doesn't just give Thor a boost, but redefines his maximum strength. He confirms that the Rune Magic he's using will grant the team "horrifying" strength and the ability to "fight beyond the point where gods and mortals tire and fall." In effect, Strange grants Thor strength beyond that of the gods, making him so powerful that when Thor finally lands a blow on Jaekelsson, he first demolishes the zombie's face, then launches him into space, where the immortal killer remains in orbit, forever ruminating on the epic loss he's been handed.
What's particularly interesting is that Rune Magic appears to be the one true key to Thor's maximum potential. In the 2004 'Disassembled' arc of Thor vol. 2 from Mike Avon Oeming and Andrea Di Vito, Thor achieves his ultimate and final form by sacrificing himself to unlock the secret of Rune Magic, becoming Rune King Thor. It seems that Strange is fully aware this is the way to enhance Thor beyond all his previous limits, hinting at the potential of Rune Magic even before fans saw its importance in Marvel's mainstream lore.
Thor has often claimed to be the 'strongest Avenger,' but when he's enhanced with Rune Magic, this claim is actually justified. Thor's usual strength - enough to battle the Hulk one-on-one - was no match for Jaekelsson, leading to grisly injury. But when enhanced by Rune Magic, Thor didn't just stand a chance, but managed to send the zombie villain into orbit. Hopefully, as Thor continues to lead Asgard in his ongoing series, fans will get to see his Rune Magic potential return, though assuredly with far less blood spilled than in Thor: Vikings' battle royale.