In the middle of the achievement above on the 3rd floor. I personally went back to it after I did the whole ship cause I had it on a different save, not recommended in some sense cause you get an "All Heart" level boost for doing it perfect, but so much harder this way. I personally, loaded up my save for level 3, cleared a path through to the final door, then ran back to create a new save. This way, if I failed, I only needed to run there in future. Otherwise, there is a checkpoint at the end of level 3 before you start the task, so if you fail, die and load the checkpoint.
To get perfect you must...
a.) Not take any damage
b.) Not hit the reverse of any moon.
A tough one in actual fact. My advice is to hold down the A and let go when you see the moon's shoot animation start and only then. Good luck guys. It can be infuriating.