Ridley (Space Coloney): Once you get less than 30 energy Ridley will fly away so you can just try to get hit and don't bother shooting him. But if you shoot him enough he'll drop the metroid, pick it back up and fly away. It is said that if you catch the metroid, then you can play the game and the metroid will be on you or it will follow you around for the whole game. I have never been able to do this before, so I'm not sure if this is actually possible.
Spore Spawn (Brinstar): If you roll into a ball and go on the ledge on the left, he won't hit you. You can shoot his little spores to get more missiles. Every time he opens up shoot the inside of him. It takes around 10 missile shots to kill him.
Kraid (Brinstar): His weak point is his mouth. Shoot his face and he'll open his mouth. When he opens his mouth shoot in it with missiles or super missiles. At first he isn't that big, but once you hit him a few times he breaks throught the ceiling and becomes bigger (he actually just stands up). The Curved things that he shoots out can be shot at to give you energy,missiles, etc. The little spiked things that he shoots out can be used to stand on.
Phantoon (Wreaked ship): As long as his eye is open and he's solid, you can hit him with missiles or charge beam (I perfer missiles). You can hide from corner to corner. He gets really angry if you use Super Missiles on him; fortunately, a charged screw attack is a good defense when he starts throwing those difficult-to-avoid attacks.
Draygon (Maridia): The easiest way to beat him is to electrocute him. First shoot all the little cannons on the wall with missiles. Then turn on your grappling beam and let him grab you. While he's holing you, grapple on to a cannon that you brok earlier. It will drain all of his energy and some of yours (about 4 or 5 packs). The other way to beat him is to shoot missiles at his stomach. To protect yourself you can roll into a ball which will lessen the damage you take.
Ridley (Norfair): If you have the space jump and screw attack, this should be no problem. Just jump up to the side opposite of where he goes up to throw his projectiles. You will be invincible. As he swoops down, stop spinning and throw as many missiles as possible. If he comes down slowly to pick you up, just stand below him and fire away. There's a magical point at the left of the platform where you can shoot up into his face while he tries to pick you up without succeeding. Also, if you have power bombs and the spring ball, you can soften him up with power bombs in the air before you begin your assault.