"This way I'll take Peach back...and I'll subdue him once and for all under my feet"
By asking for help from the Mushroom Kingdom sorceress, Mario thought he could solve the problem of the size and strength difference between him and the dragon but he didn't think that the spell wouldn't also make the villain less muscular. In fact, facing each other in a rather normal fight, Mario gets the worst of it right away and Bowser teaches him to stay in his place, subduing him with his muscles, with his own strength and... above all, humiliating him by rubbing his gigantic cock between his tight buttocks in a Boston Crab. He didn't expect Mario to like it, but apparently the plumber has some secrets that he also hides from Princess Peach.
Initially I wanted a Luigi vs Bowser...but I kinda ship them, so it's better two enemies :P