Once Kraid's crumbled into the ground, go through the door on the right of his chamber to find the all-important Varia Suit. Once you've got it, head back to the elevator leading down to Norfair (don't forget to grab Energy Tank no. 1 (Brinstar) on the way). Once you reach the elevator ride down into Norfair, and get ready for a hunt for item power-ups.
We'll first go looking for the Speed Booster.Go through the door down on the right below the elevator to find a huge heated cavern. You won't take damage anymore as the Varia Suit will protect Samus. Proceed through the room while blasting enemies, eventually you'll reach a door leading to a second lava cavern. In this room you can find missile no. 10 (Norfair). In the next room lava will be slowly rising up from the ground, quickly head through and try to avoid all the enemies around.
The next cavern you'll enter will be filled with green terrain, here you can find missile no. 2 (Norfair). Go over to the small platform in the center of the room and bomb it open. Drop down below and traverse through the narrow tunnels below, just watch out for the orange Geemers. Once at the bottom, go through the door in the floor.
Go down the shaft, the door halfway down contains a few enemies that'll drop several energy and weapon refills, once you're ready, go down through the door at the base of the shaft. Keep going right through the next few corridors, eventually you'll reach a shaft leading up. Climb up the shaft, and watch out for the heads in the walls that spit blue fire. In the next room, you'll have to ride across small green platforms to avoid the lava below.
Once you reach the far side, go through the door reach another shaft (this game's full of them). Go up to the very top while blasting the dragons and Multiviolas along the way. Once at the top go through the door to find yourself back in the large green cavern, this time you'll be in the top right corner of the room. Go up to the door above you and head through. Beyond here is the Speed Booster. Don't forget to grab missile no. 3 (Norfair) just outside the Speed Booster's room.
Once you've got the Speed Booster, retrace your steps back to the main elevator of Norfair. Once there, go through the door down on the left from the elevator. Run through the corridor, passing all the closing shutters - the Speed Booster will get you straight through. Go through the door at the end of the corridor, in the next room you'll be able to find the Ice Beam here.
Once you've got the Ice Beam, head to the top of the shaft outside the Ice Beam's room and go through the door up there. Drop down the next shaft, you'll end up back in the corridor with the closing shutters. Blow open the small tunnel in the roof, blast the Geemer in there before going through. Roll through the tunnel and head back up to Brinstar.
Once you reach Brinstar, go all the way back to the huge orange shaft. Now that you have the Ice Beam, you'll be able to freeze the Ripper flying in the air to climb back up. Freeze them and climb up the shaft, keep climbing up to the very top of the shaft then go through the door at the top. Head through the spike-filled room, once you reach the next shaft shoot the floor open to eventually find Power Bomb no. 1 (Brinstar) and missile no. 11 (Brinstar).
Once you've found them, go back to the shaft and go up to the very top. You can find Power Bomb no. 2 (Brinstar) in the room on the left below the elevator, but once you're ready take the elevator up to Crateria.
Taking this path back to Crateria, you can head back over to the other main elevator of Brinstar on the far left of Crateria, this way you can pick up a load of items before heading back to Norfair. In Crateria, you should be able to find Power Bomb no. 1 (Crateria), missiles no. 7 and 8 (Crateria) and Energy Tank no. 2 (Crateria).
Going down to Norfair via Brinstar you should be able to pick up missiles no. 7 and 8 (Brinstar), Super Missiles no. 2 and 3 (Brinstar), Reserve Tank no. 1 (Brinstar), Energy Tank no. 2 (Brinstar) and Power Bombs no. 3, 4 and 5 (you'll pay the Etecoons a visit when looking for no. 4). You can also grab missile no. 12 (Brinstar) near Kraid's room as well.
Phew, with this amount of weaponry you should have enough to take Crocomire for a swim. Once you head back down to Norfair, first visit the Map room down the shaft with the main elevator (blow the door open with a Power Bomb), then go back into the corridor just below the elevator that contains the closing shutters. Run past the shutters, then use a Power Bomb to blow open the hole at the end of the corridor.
Drop down and go into the large cavern below. Blast all the fiery enemies, then head through the door on the far left side. In this next shaft you can pick up missile no. 6 (Norfair) in the opposite wall, but if you're having too much trouble reaching it come back once you've found the Space Jump. Drop down to the bottom of the shaft and head through the door there. In the next massive corridor, dash to the right until the Speed Booster kicks in, you'll then blast through multiple enemies along the way, after breaking through a few rock walls you'll land in a room with a door in the floor.
You can save your game in a Save room further up the shaft, but once you're ready go through the door in the floor to face Crocomire. Refer to the bosses guide to melt his hide.