Your first battle against Bowser Jr. isn’t tough at all. Grab the Propeller Block and zoom into the air above the buzzing bite-sized Bowser. Dodge his fireball attacks by staying in the air and drill down on his head to knock him hard!
When his fireballs hit the ground, they’ll light it up like a bonfire, so stay away from the flames, and wait for Bowser Jr. to fly overhead again. When he does, jump back into the air, zoom over him, and hit him over his head a second time. Repeat this one more time to finally defeat him.