The turtle from SMB3 has returned! Boom-Boom smashes the ground as he walks across the stage, trying to hit you. Just jump on him, and do that three times to defeat. But after one stomp, Boom-Boom reverts into his shell, so don't hit him like that.
Larry is fought on a basic arena with ditches with spikes on each side. He'll also shoot fireballs from his wand. Jump on him, and do that three times to defeat him.
The King of Koopas thought he'd get an early battle with Mario. Bowser will spit fireballs while moving back and forth. To defeat him, jump over or dash under him and hit the red switch on the other side of the arena, which will cause the bridge to collasp. Bowser can also be defeated by fireballs.
Birdo got hired by Bowser to defeat Mario when he arrived. She has the same technique as in SMB2, but you don't defeat her the same way. She'll spit a Green Shell after a bit, and you must stomp on the shell and fire it back at her. Before you fight her, she says "Forgive me, plumber.", but only if the player is playing as Mario or Luigi.
Roy has returned! When he lands from a jump, it will stun you, no matter where you are. Just stomp on him to defeat. Do this three times to defeat.