The following is an in-depth guide for the seventh area of Metroid II: Return of Samus.
Go all the way up and then go left so you can fight a Gamma Metroid. Next, go left to the next room and fall down a bit and go left so you can find a Missile Expansion. Go back to the right a little bit and then go upwards hugging that right wall. Keep space jumping going upwards and eventually you'll find a Missile Expansion, a Gamma metroid and a Heal Pool that you'll have to use the spider ball to get. After you get all of this, go back down a little bit until you find an immediate path right. Keep going right to get to a Zeta metroid. It'll take 20 missiles In the face, back or bottom to kill. Go back out to the left outside of this hallway to get back to the main area. You can either Space Jump upwards to the left or spider ball stick to the left wall. Go up this wall. You might find a missile heal pool on your way up. As you're going up, you'll find a weird spot in the wall. Put a bomb there and then quickly get out of the way to bomb it and get inside. In here you'll find another Zeta Metroid. Use the Spider Ball in the ceiling to get up, then bomb through the ceiling and go right to get the Screw Attack. This makes you invincible when you spin jump. Go back out, back down and back to the right to get back to the large open area. Use the spider ball or the space jump to climb all the way up and land on the left ledge. Keep heading to the left until you find a spot in the ground you can fall down. This area is completely in the dark, so I put a map for you to follow. Otherwise, follow these directions carefully.
After you drop all the way down, get into your morph ball, ram into the right wall, and jump while holding right. Once you stop rolling to the right, (You can guarantee this by either waiting a while or placing bombs to see if your moving) bomb the ceiling. After you bomb the ceiling, jump up out of the hole or stick to the walls with the spider ball. Here you'll get an Energy Tank. Next, drop down, get into the morph ball, hold left, and bomb the floor a couple of times. After that, go all the way to the left until you drop down. Once you drop down, run into the wall to the right, bomb the floor a couple times, and then bomb through the right wall and go all the way to the right to get the missile expansion. Then use the spider ball to the left bombing through the walls to get out of the holes. Once you're out, jump to the left and stick to the far left wall and go up. On the first block on the ceiling, bomb it, then go inside. Here you'll get a Missile Expansion and Energy Tank. At the end of the hallway, bomb the last part to get out. Keep holding on this wall to get to the right side. Keep heading to the right and you'll find a Gamma Metroid to kill. Go all the way back out to the left over the holes until you can't go further left. Fall down this shaft until you find the first door to the left. This is an optional section if you want to change your beam. I personally wouldn't change it if I were you, but if you want to there's the option. Go through it and down and you'll see 4 red doors, in each of them is one of the four beams. Choose which ever one you want and go to the bottom and through the very bottom path. After you do that, go up and through the first left door, or if you skipped that go down and through the first left door. Go all the way to the left and fight the Gamma Metroid that you come to. After that of back to the right and out of the hallway, then go all the way down the shaft to the first left door you come to. In here there is a Zeta Metroid to fight. Go back out to the right to the shaft from earlier, go up to the first right path out of the shaft and you'll find a save point. Go to the right to find a big shaft. Go to the very bottom of this shaft and take the door to the right. Go all the way to the right of this area to reach another shaft. Go to the bottom of this shaft and go right. After that head to the bottom of this area to enter Phase 8.