when you get to the half way point, you will dodge 2 fireballs and go down 3 steps. then,
jump up onto the platform with the moving thing on it, then, quickly, jump onto the
platform with the koopa on, jump on kill it and quickly and try and bounce above the top
platform which is moving up and down. when you are on it, run all the way to the right and
you will find the coin.

(then go through the door)