How could the Federation create such a sight? The pained and thoroughly-beaten Nightmare still has some life left in it, despite losing its protective faceplate and having that monstrous (and screeching) face exposed. Upon picking itself up off the floor, the shrieking monster will fire up the gravity well projector yet again. Only this time Samus has the Gravity Suit equipped, so no more loopy gravity. Missiles won't work while the gravity well projector is active however, so it needs to be frozen again.
Once airborne, Nightmare will deploy larger purple orbs against you. This time they travel slower, but continuously home-in on Samus' position. Run clear or use the Sensemove if the orbs travel too close for comfort. You'll need to keep clear of the orbs until they eventually dissipate on their own. Open up on Nightmare straight away with rapid Plasma Beam shots - ironically your beam will be sucked straight into the gravity well's vortex. Stay on the ground floor and keep pummeling the device until it freezes.
With gravity restored to normal it's time to open up on that green face with Missiles (and Super Missiles if you can manage). You should probably abandon the lower floor for higher ground as Nightmare still tends to hit the ground and slither towards Samus. Sometimes during the battle, Nightmare will rather comically lose its balance and fall in a heap. Try to sneak in a few easy hits once it crash-lands. The Plasma Beam can also harm Nightmare's face without the faceplate protecting it. If you ever need to restore health with Concentration, hide in one of the shafts for protection before using the technique. Just keep an eye out for any rouge purple orbs.
Nightmare will eventually deploy a very nasty reddish-purple laser from its mouth, sweeping it around the chamber. Either leap up the chamber, drop down a level or simply Space Jump over the beam as it passes by. Keep moving around at all times to avoid the constant homing purple orbs, and don't stop to fire at Nightmare until you're clear of the barrage. Nightmare may also leap down atop one of the higher platforms and slither towards you, just take care when flying around with the Space Jump as some of the orbs may still be active.
After a few cycles of the gravity well freezing and unfreezing, Nightmare will exude a massive energy vortex at Samus. Leap clear and keep moving. The vortex will remain where it landed for some time, and all your projectile shots will be sucked into the black hole. You can either dodge everything Nightmare throws at you and wait for the vortex to wane and dissipate, or you can try to position Nightmare between Samus and the vortex - fire from here and your shots will strike Nightmare on their way towards the vortex.
Keep clear of the vortex as it shrinks - once it dissipates fully, a massive and completely blinding blast of energy will light up the room. Don't let this flashbang effect take you off-guard. Once Nightmare's health is considerably low, it'll continue to use the black hole vortex so as to increase its chances of survival. Be patient and avoid everything it throws at you. Once the vortex explodes you might want to use the Plasma Beam to score hits faster and more rapidly than Missiles. Drain its health fully to finally put this beast to rest.
Playing against Nightmare on Hard mode won't be as challenging as Ridley, but don't underestimate his attacks - they can still easily finish unwary travellers.