This stage is Fire Bros. Hideout #3 (6-B). This complete walkthrough will provide you with a 100% video playthrough. To access this stage, look for a pictured patch of ground on the map next to 6-2. This stage is pretty crazy where you have to battle with a bunch of hardcore enemies including two Hammer Bros., two Boomerang Bros. and one Fire Bro. Your best bet is to enter the stage with projectile weapons yourself such as the Boomerang and Fire Flower.
An alternative way is to let their own projectiles hit each other, however this method is a little bit more risky.