This level is an automatically scrolling level. There's a tank procession and you have to get past it. The blasters fire Bullet Bills at you and soldiers walk in formation on the ground. Pound the crates on the tanks so you can reach things. Get the Fire Flower and hit the far right item block on a different tank to get a Cannon Box. It will automatically fire a projectile in the direction you're facing. Charge the shot by holding (X) or (Y) to increase the range. Try not to run into any enemies so you can keep it. You'll need it. Get the Red Coins if you like. The blasters on brick blocks will turn into Cannon Boxes if you destroy the brick blocks.
The tanks will disappear for a while as you go around a corner. There are Fire Piranha Plants here. In the middle of three of them is the first Green Star. Soon, you'll come across a tank with just a crate on it. The second Star Coin is in it. There's a tank with some grating on it. Destroy the crates and jump onto the grating to find the stamp. At the end of the procession, you'll find a wall with Bowser's seal. Use the Cannon Box to break the wall for the third Star Coin. Go through the pipe and you'll battle Boom Boom. He'll spin around, then stop when he's dizzy. Stomp him, and he'll go into a shell cycle. Stomp him again and he'll go into a shell cycle while invisible. He'll also try to attack while invisible. After three hits, he will be defeated. Coins and a warp box will appear. This will take you to the flagpole, where the yellow Sprixie Princess is trapped in a jar. Land on the flagpole to break the jar. She'll get a hammer and build a clear pipe to World 3.
The stamp is of Boom Boom.