Super Sunny Island

You can access the first boss once you have 15 special coins. Enter his level to the right of 19-1 with the evil pumpkin face.

This boss is about as simple as boss fights get. Just avoid his slow moving projectiles and wait for him to try and stomp on you. Once he is on ground level, shoot him with . Rinse and repeat until he is down, should take no more than 5-6 "stomps" worth of shooting.