Lets face it, Kirk CAN beat up anyone because he is Kirk from Star Trek.
Not really. Remember, Roddenberry's whole purpose with Star Trek
was to get away with the cartoony, larger-than-life fantasies of most science fiction on TV and approach the genre as a naturalistic adult drama, with more grounded and human characters. Kirk wasn't supposed to be some perfect superhero, but a relatable everyman with flaws and doubts and vulnerabilities and limitations. He generally did win his fights, but often with difficulty, getting pretty beat up in the process. And there were a number of fights he didn't
win. Spock totally wiped the floor with him in "This Side of Paradise" and "Amok Time." The Air Force guards took him down in "Tomorrow is Yesterday." Cloud William beat him easily in "The Omega Glory." So he wasn't some unbeatable fantasy hero.
He was the only one with willpower enough to beat the spores in "This Side of Paradise". He won a fist fight with a genetically-engineered superman in "Space Seed". Earlier in the same episode, he remained concious longer than anyone else when the oxygen was cut off to the bridge.
Kirk was mortal in some episodes, but not all.