Next to level 5-5 on the World Map is a small patch of rocks you can investigate. Doing so reveals a hidden pipe that takes you to an underground area where you can take on 5-B.
This battle gives you 100 game seconds to clear it. A massive Goomba Tower confronts you on an island surrounded by lava, and at the top of the stack is a Fire Bro. You must use the rising pillars on the island to get high enough to stomp on the Fire Bro and put the battle to a swift end. A Green Star is your reward.