Once you unlock World Castle-4: Boiling Blue Bully Belt, you should see some stones to the right of it, on the ground. Go to those stones to see a button prompt, so hit the button to make a pipe appear, which will take you to the location of this hideout. While the fight you will have to do is simple, you are going to have to deal with a total of seven Fire Bros here.
Naturally, things are definitely crowded at the start of the battle, but this can be easy or hard, depending on a few factors. If you have a ranged power-up, like the Fire Flower or Boomerang Flower, then you should have an easier time here. Also, the Fire Bros can actually take each other out with their own projectiles, so there’s a possibility of that happening. If you have a normal power-up or none at all, then you just need to be extremely careful. Defeat all seven enemies and you will net yourself another Green Star.