With the help of Minion #1 Tavros, Bowser found his way to the Sensoriums and wasted no time recreating huge swaths of the Mushroom Kingdom, cycling through various locales as fast as he could. After wasting copious amounts of time going HOLY CRAP THIS IS AMAZING GWAHAHAHAHA, the Koopa King calmed down enough to remember that he had a duel with a certain book Overlord('s minions) to think about. Bowser decided to go with a flat platform of rock inexplicably floating above a sea of lava, deep beneath the earth- his favored battlefield, back when he'd stolen those 120 stars and sealed everyone in those paintings (just... you know, without the bombs that Mario had thrown him into).
And then it was time to warm up. Bowser cracked his knuckles...
A few minutes later, the Koopa King skidded backwards on all fours, his clawed feet leaving deep gashes in the rock as he braced himself. A split-second later, Bowser catapulted forward with a roar, swinging at his opponent with enough force to smash boulders- and only found air as the unassuming, somewhat chubby mustachioed man in overalls jumped out of the way with impossible speed. Grim-faced, the plumber flicked his wrist, flinging three fireballs before he even touched the ground.
Bowser shielded himself against the first two with his forearms. With a laugh, the hulking dragon turtle straightened up and contemptuously swatted away the last one- "ARGH!!"- before immediately regretting it because OH GOD that stings. Frantically waving his injured hand to cool it down, Bowser snapped his fingers with his GOOD hand. "Goombas! To me!!"
"GOO-RAH!" A horde of squat fungoids dashed toward the plumber just as Bowser fell back, but the human turned on a dime and jumped onto the first Goomba, stomping it flat and catapulting himself onto the next one. In fact, the plumber's feet never touched the ground as he worked his way through the horde, right toward Bowser-
- and the Koopa King immediately sent a stream of fire in Mario's direction. The battle was once again joined.