This level can be found as soon as you enter World 6. Look for the pictured patch of ground on the map to discover the pipe that leads to it.
It’s not just Fire Bros. in this hideout! Two Hammer Bros. and two Boomerang Bros. ambush you alongside a single Fire Bro. Worse still, you have only a 100 ticks of the clock to beat them all. It’s tough at the start of the fight, with so many attacks being thrown around, but there is one thing the baddies didn’t take into account: They can hit each other. It’s possible that they’ll knock each other out before you can get to them—if you dodge at the right moment. However you do it, clearing the Fire Bros. hideout earns you a Green Star.