This is how I beat most bosses.
Plant boss, sector two, item: Highjump, jumpball.
When u jump down, start charging a beam instantly. When the beast jumps, keep yourself outta its way while charging that beam. Don't use missiles until the second stage.
Anyway, when he jumps straight up, run under him and release the beam into him.
Then start running and charging. When he loses all three segments he will get faster but only jumps once before you can attack him. Just fire about 3 or 4 (Depends on how fast you are) missile into him once, get away from him and do it again. Then he should turn into Core-X!

Serris, sector four, item: Speed boost
Well, there isn't much to tell except the place where you can hit him the best from is two blocks from the right under water. I'll find a screen. The only way you can get hurt there is whebn he is super speed he might run into you twice in a row!
But that happens a max of 2 times... But from there you can hit him from almost every angle.

Yakuza, reactor silo, main deck, Item: Spacejump.
He is pretty hard.

First, you need to run to the corner. Make sure he doesn't grab you. While he is bouncing off walls, just stay morphed in the corner. When He pauses a second time, hold R and run under him. Fire about 4 or 5 missiles into his mouth then roll into a ball and run for a corner. When his legs break off, stay in the corner and fire missiles diagonally, they have a 60% chance of hitting him, and a 90% chance of you not getting hit. Keeop firing at his head until he explodes and turns into the Core-X.

Ridley-X, Sector 1, Item: Screw Attack
Now this one I haven't beaten a lot.

First, he never changes, so I will tell you how to beat the zero mission one as well.

Ok, first, for zero mission, he will break the edges of the floor, so press up against the door.
In fusion, press up against the wall.

Now, in both, fire diagonaly up at his chest, don't stop firing. If he fires fireballs at you, there is a chance your missiles will hit them. If he grabs you, keep firing. He will drop you and you can still cause damage. Basically that's all. He dies quite soon and in fusion you get screww attack!

P.S Don't forget, never leave the wall in both games. Bottom right.

SA-X, Main deck, operations deck, Item: None.

You really need to get the hang of space jump.

What to do:
As soon as you enter the observation deck, start charging a beam whilst space jumping to the red hatch (Top left hand corner.)
When the screen shakes, jump down and run towards the locked hatch (Bottom right) when you reach it, turn around. The Sa-X should be behind you. release the beam, and automatically after, jump and charge. When charging, just jump aroun with screw attack, when done charging, jump to the ground and run to the locked hatch again.
*Note* You need to be fast. Real fast.

Omega Metroid, Main deck, docking bays, Item: Temporary Ice beam
The easiest boss. The only thing you have to worry bout is the time limit.

What to do:

As soon as you enter the docking bays, run towards the metroid skin.

Then run back the way you came and the metroid will jump outta the wall. run up to it and let yourself get hurt.

Then, when it attacks the SA-X, and you absorb it and get ice beam, just hold onto L or R, can't remember which,
then fire at it's chest until it rears its hand back. then run away until it slashes, then run back
Lather, rinse and repeat.

That's all right now.. :metroid:

I posted this someplace else, but I guess I will repeat myself here too.

Nightmare. Beating it under 40 seconds (with practice, like myself).

When you drop down into Nightmare's chamber, go to the center right of the room where he begins to come up in a transparent sort of state. He will not hurt you until he is fully in the room, fully visible. As he begins to get higher position yourelf directly under his gravity controller. Charge your beam and aim up. The split second that he becomes fully visible and vulnerable, release your charged shot. This will damage him greatly, and cause him to turn on his gravity controller. Quickly charge your shot again, and release it on the controller again. This will disable the controller. If you are fast, it will be like he never turned it on. Roll into a ball, go to the wall with the ladder, unmorph and jump up to the middle of the ladder. Charge your beam and wait for his face to blow off. After releasing your charged shot, unload missles into his face. Then, Core X, ect ect.

I have pretty fast fingers so I am trying for a 30 second Nightmare beating. I have a save file on my GBA game just for trying over and over .. haha. Its fun.

Serris. Basically, there is a spot in its lair where it will not touch you ever. So you can just keep firing at him and he will have no chance to hurt you as long as you dont move. Its the second tile of the room, underwater, just after the door. So you have to run up to the platforms to awake him, then jump back down into the water. Go to the second tile, and crouch. You can charge you beam or use missles to kill it. Make sure you are not too far to the right or you could get hit by some of his attacks. And if you are too far to the left he will come up from under you. So you have to find the correct place!

B.O.X Security Robot, first phase. Easier than heck.

All you basically need to do is (after getting it to chase after you) jump up to the ladder, hang on, and position yourself directly over it. Aim down and unload missles into it. When it closes up and lets the little ball thing out, position yourself directly over where the ball stops (while still on the ladder). It will explode into fire columns, just stay where you are and they wont hit you. Get over B.O.X. again and unload more missles. That should take care of it.

It is possible to defeat B.O.X. after only two tries. You gotta get four missles into it and then another three. Easy if you are fast.

Thanks, can I add that to my site