Whoa, what's this guy doing here? The Nightmare is back from Metroid Fusion, sporting some new tricks along with a few surprises. True to form, Nightmare immediately activates its gravity well projector to seriously mess up the gravity in the chamber. As you'd expect, the gravity manipulation reduces Samus' movement to a slow crawl while Nightmare remains unaffected. This is going to be a very hard battle with plenty of trial-and-error.
Stay at the top of the chamber and watch out for a volley of purple orbs fired from Nightmare's arms. The gravity well won't stop Samus' Sensemove, so use it to steer clear as the orbs travel close. Don't waste any time in opening up with your Plasma Beam; try to score charged hits as much as possible. At this point Samus will aim for the gravity well projector - you need to take that thing out to regain Samus' normal agility.
As you continue to fire at the projector, watch out for Nightmare's nasty sweeping laser. If orange-yellow lights begin appearing across its arms, very quickly abandon your position and leap off the platform to the lower level below. Nightmare will sweep its lasers right around the chamber in a circular motion, and thanks to the low gravity you can't safely leap over the top of the blast easily (or sneak under it with the Morph Ball). You can try to madily tap jump to Space Jump over the beam, but it's extremely risky. A good tip is to stay near the connecting vertical shaft leading down to the floor below and simply hop into the hole when the laser deploys (thanks The Ogvernment!). Do not take a hit from this laser on Hard mode - one strike and you're out.
Be careful if the lasers are still active as Nightmare slowly lowers down after Samus. Once Nightmare catches up with you, watch out for any further purple orb volleys. Take extreme care if you're down at the bottom of the chamber - Nightmare has a nasty tendency to slam itself onto the ground and slither over to your position - go nuts with the Sensemove to leap clear.
If you do find yourself on the ground floor, immediately run for the narrow shaft in the wall leading to the next floor above. Either use the Grapple Beam to hoist yourself up or slowly Space Jump up the shaft to ascend the chamber (tap jump very rapidly when initiating your climb up these shafts or Samus will miss the wall and slump to the ground). If you ever need to use Concentration, fall down a level or two (if possible) and hide inside one of these connecting wall-jump shafts before performing the technique there.
Once you've climbed up one floor above the ground floor, stay there. Don't bother climbing up any further - stay near the wall-jump shaft and alternate between this floor and the ground floor when avoiding Nightmare's laser.
Eventually with enough punishment your Plasma Beam will freeze the gravity well projector, sending it temporarily offline. With gravity restored, now it's time to take action. Enter first-person view and lock onto Nightmare's faceplate. Open up with your Missiles (try a Super Missile if you're feeling lucky) - just watch out for any wayward purple orbs coming your way. Also keep an eye out for the sweeping laser - now you can safely Space Jump over the blast.
If you're playing on Hard mode, try to score Super Missile hits at all times (and charge up your beam first before entering first-person view, then keep holding down the fire button (1) and release to deploy a quick Super Missile). Use Concentration immediately upon scoring a hit to replenish your Missile supply before Nightmare attacks again.
Damaging Nightmare's faceplate will start chipping away at its health, but its attack patterns won't slow down. If you're on the ground floor, quickly climb up to the next floor to avoid Nightmare's weird slithering motions.
However, on higher floors watch out if Nightmare leaps down atop your platform. It'll proceed to stomp around to your position to try and ram Samus off the edge. You can either climb down a level to avoid it, or use the Space Jump to fly around the monster as it passes (so long as the gravity is normal). If the gravity is normal, wait for Nightmare to leap down on your platform, then try to score a quick Super Missile hit on its face.
With enough missile damage to its faceplate (which by now is sporting visible cracks), Nightmare will unfreeze its gravity well projector and fire up the beast again. Once the gravity's gone loopy you'll need to unload the Plasma Beam into the projector to send it offline.
Continue to use the Sensemove against purple orbs and leap down a level if Nightmare's sweeping laser or stomping run gives you trouble. Open up with your Missiles or Super Missiles once the gravity well's frozen again. By now you should be adept at avoiding Nightmare's attack cycles, just keep unloading Missiles into the faceplate and keep the gravity well projector frozen to finish this beast for the second (well, chronologically first) time.