Answer:
At the end, of the first level of World 2, you will see a pipe in the sky that is unreachable. But just before it, there are two blocks that are hidden, with one being lower than the other. Hit them both, from the ground, the lowest one will give you a coin, the second will give you a green bouncy thing (or a mushroom if you are small Mario at the time). Then jump from the shorter block, to the higher one, and then onto the green pipe in the sky and go down it, which instantly takes you to Koppa the Devastator.