Magmoor Caverns
This is the final beam weapon and, other than the corresponding Missile Combo, the final upgrade before facing the boss of Phazon Mines. Aside from Chozo Artifacts, you've gotten nearly everything there is to get outside the mines, and we'll be wrapping up what's left here.
Geothermal Core
Return to the Geothermal Core, and get to the entrance from South Core Tunnel. Scale the ledges near the door, then locate a Grapple Point under a large circular platform attached to a pillar in the middle of the room. Swing to another circular platform across the room. Note, getting this the first time is fairly easy, but later when the position of the platforms has changed you may find it more difficult. If you have trouble, go two rooms away and come back to return the platforms to their original positions.
There is another platform nearby, but before going there use the Spinner on the platform you're on to raise the upper half. You'll be making the rounds of platforms here and if you miss a Spinner you'll get stuck somewhere further on. When you get to the second platform, activate the Spinner and jump to the upper half of the first platform.
Jump from there to a third platform (the one with the Grapple Point under it), then activate the spinner and follow the Spider Ball track to the upper half. Now jump to the upper half of the second platform. It looks like all that's left to do here is to activate a Morph Ball slot here, but when you do some machines at the tops of the pillars lift the ceiling and reveal a huge Spider Ball maze which circles the room. Drop bombs to clear the Plated Parasites when needed, but other than that you just have to use the skills you've learned up to now and going into detail won't be much help.
When you've circled the room you land on a ledge with a door, so go through.
Plasma Processing
This floor of this room is mostly lava, so pick your way carefully to the central island and pick up the Plasma Beam. This is the most powerful Beam Weapon in the game, something that won't be entirely obvious because you can already one-shot many enemies. So try it out on something like a Sheegoth to truly appreciate what you've got now.
There is a red door to get out, and the Plasma Beam opens these as well as Blue doors.