In GMFC Compound, your supposed to witness a cutscene and then blast a yellow crate and recieve the Missile Launcher. However, right before the "invisible wall" leading to activate it, there is a boot shaped formation. By jumping onto it, you will be propelled onto the ledge above, allowing you to skip the activation of the cutscene and go straight to the Missile Launcher. After getting it and then activating the cutscene, glitchingly the yellow crate reapperars, with ANOTHER Missile Launcher. Geting this 1, and the remaining 99 items in the game will allow you to have 101% items at the end of the game (your Missile count doesn't go above 255 however).
This is a very simple way to despose of metriods, and you waist no beam ammo. All you need is the power beam, and a door. Of course, the metriod helps too. First, shoot the metroid with the beam, to make it go after you. Face it, with your back to the door. Wait, just wait. When the metroid charges you, dodge to the side. Hint, you can only dodge while locked on. If the metroid hits the door or near it, it will explode.