The first part of the level is very linear. Just watch out for the purple birds that charge at you and the spiked turtles. But then there is a fork. It is advisable to take the path on the left. The crystal is in fact on this side. Along the way there is also a box (!), which when shot will spawn two more boxes along the way to the right, just before where the two paths rejoin. After rejoining, the path is linear again to the exit room.
There is a bonus area shortly after where the two paths rejoin. Overcoming it is easy, but if you want to break all the boxes, you have to solve a small puzzle. In fact, you will notice some invisible boxes at the top, just before a large pile of metal crates, which you will have to crawl under. Then there are three bouncing boxes suspended in the void. Use them to overcome the gap, but don't break them. A little further on is a (!) box, which when shot will bring up the invisible boxes you just saw. Go back using the bouncin boxes, but move so as to leave at least the central one intact. Break the boxes that have appeared and go back using the last bouncing box. The platform that will take you back to the level is just ahead.
Getting the gem[edit]
There is only one gem in this level, which can be obtained by breaking all the boxes. Nothing too complicated, there are no hidden boxes. However, you will have to pass the bonus area by breaking all the boxes as described above and take both paths after the fork (better the one on the left first, to hit the (!) box). This means you'll also have to walk towards the screen, which is a bit trickier.