Description:
An Underworld equivalent of a bio-weapon, Nightmare is designed to destroy and not much else. It is sentient, but it acts as though it has no will of its own, blindly obeying its creator Mundus. Any opponent trapped inside its recesses is doomed to an eternal torment by their own nightmares...

Taking over the task of killing Dante after the defeat of the Griffon, Nightmare is one tough cookie. It has two forms, a pile of goo and its attack form. Its goo form is simply that: a formless pile of green muck and indeterminable chunks moving slowly on the floor towards you. Its other form is more solid. It becomes a large creature that resembles an old steam train made out of otherworldly flesh and moves on the goo like substance of its previous form. In this form it is stationary, but hoo boy it makes up for that.

Sounds:
Apart from the various explosions and weird whirring noises of its many devices, Nightmare is surprisingly silent. It never says anything throughout the course of the game.

Attacks:
I will divide the attacks into the various forms:Liquidform attacks:
-ice slugs (light-medium, far): Nightmare sends out several black slug-like things that swiftly track Dante. If they latch onto him, they explode in an icy blast, and can freeze Dante if enough hit him.

-absorb (no damage, close-medium): Nightmare creates a small puddle underneath Dante's feet and tries to swallow him up.

-net (no damage, close): Nightmare jumps up onto the roof and drops a large net around Dante.

-smash (heavy, close): After using the net, Nightmare drops its entire weight on Dante.

Solidform attacks:
-mini guns (light-medium, any range): Nightmare summons four black orbs around it, which shoot multiple bursts of silver energy at Dante, like a machine gun.

-poke (medium-heavy, close): Nightmare sticks out a sharp pointy thing like a finger from its body and tries to pierce Dante repeatedly with it.

-boomerang (medium-heavy, close-medium): Nightmare detaches a large part of itself that begins to fly about the room in a set pattern, usually around Nightmare itself.

-ice beam (heavy, close-medium): Nightmare charges up white energy in its mouth and releases a thick white beam. If it hits Dante, it freezes him in place for a while and prevents him from attacking.

-homing missiles (light-heavy, close-medium): Nightmare creates a multitude of black slug like creatures, but this time they fly towards Dante to damage but not freeze him.

-energy drain (no damage, anywhere): Nightmare drains Dante's DT gauge into its body.

Strategy:
More than any other boss in the entire game, Nightmare is loathed. And with good reason. He is one of the most annoying bosses in the game, with very high defense and more attacks than any other (yes, even more than Mundus) Combine with the fact that he can be hit all the time, and that Holy Water is strangely ineffective after his first meeting, and that he is the only boss that sucks your DT gauge, and the small arena of later fights, and his infernal ice beam, and million other things, and he can make you tear your hair out in frustration. I have seen more requests on how to hand Nightmare's non-existent behind to it more than any other boss.

The key to defeating Nightmare is to make it solid. The thing takes no damage whatsoever in its liquid form. To make it solid, he must be bathed in light. Every time you fight it there are dials on the wall. Hit the dials until they light up, and light will fill the room, making Nightmare solid. This lighting scheme is not permanent, so you will have to constantly have to redial the lights. Get used to doing that. A lot. In its solid for, the only way to defeat it is to hit one of its four cores when he opens them up. They are glowing balls, which change color from time to time

In its liquid form, Nightmare slithers around slowly, and sends a few slugs after you. It also tries to absorb you from time to time. To avoid the slugs, roll, jump or attack them. It is better to avoid them than attack, because you could get absorbed. To avoid being absorbed, jump, roll or run out of the way. It can only absorb a stationary target. Nightmare does NOT use the absorption technique when it is on the roof; however it will still use slugs before it jumps to torture you more. To avoid the net, you must stay as far away as possible from Nightmare. If he does get you with it, then hack and slash through the net, because cutting through is the only way to avoid the smash. It only goes to the roof in the third battle, so you won't have to worry about that at the beginning.

Light the dials up while avoiding its basic attacks and Nightmare will solidify in a surge of lightning. It will now remain in the same spot until the lights go off. It can rotate, but it rarely does that. Now, to avoid his attacks, you have to know when they are coming. Nightmare almost always attacks when he has an orb showing. You will be using guns for this battle (preferably the Handguns-you heard me-, Grenadegun or Nightmare-Beta. Shotgun seems ineffective this time around), and your choice of the Alastor or Ifrit. If you are using Alastor, use a Round trip just
before it exposes its core, then DT and smash its cores with Air Raid. Vortex Lv2 is also a good technique if you can get the aim right. Dante will grind into the core while simultaneously deflecting attacks. If you have the Ifrit gauntlets on, jump kick to the core, then DT and use Inferno or stand at a distance and use Meteor. Inferno is all round better, both for damage and more importantly, speed. Inferno comes out faster than a fully charged Meteor shot. Sparda is fairly pointless since it can't DT and Force Edge...well, you know why.

Nightmare's absorption technique is unique for a number of reasons. When Dante is absorbed, he enters Nightmare's murky recesses and becomes trapped in his own nightmare. You will face off against a few small Sargassos and a large one. Once they fall, you will have to fight one of the earlier bosses. The first time you
encounter Nightmare, it will be Phantom who you must face. The second time, it is a non-flying Griffon. The third time, it is a slightly weaker version of Nero Angelo.The Phantom is not quite as dangerous as he used to be, The Griffon never leaves the ground to attack, and Nero takes more damage than the norm. Use whatever weapons you feel fit to defeat the bosses, but I would recommend Sparda for Nero once you use up your DT. Longer range than Nero's blade and increased damage too. Once you defeat the boss, step on the glowing circle to leave. Dante will reappear in the normal plane and cut through Nightmare, reducing the bioweapon's health by a whopping 25%! That's what you get for thinking you can absorb the son of Sparda! I would recommend getting absorbed at least once every battle, especially as he gets harder to kill as the difficulty levels get higher.

To avoid his poke attacks, stay out of range. Nightmare uses this attack when Dante stays too close for its comfort, like a counterattack. His boomerang too is an attack that is based on proximity. It's fairly easy to see coming, and never leaves its trajectory once out until it goes back to Nightmare. Stay out of range of the projectile while attacking. The homing missiles too can be avoided by rolling or jumping out of the way, but you can deflect them back at Nightmare to damage his core. His energy drain is unstoppable; all you can do is pound the core to force it
to close it. The ice beam is a deadly attack, but the key is to stay behind Nightmare (The large knob is Nightmare's "front") and wail on his core from there. The miniguns are often used when Nightmare has no core showing. Avoid them by constant, erratic movement so they can not lock on. The third time you fight it, be careful.

The arena is very small; so small, in fact, that there is hardly any room to avoid its attacks. It also gains a more
powerful icebeam, which it can use to track heights. When you beat up on it enough, a cutscene occurs. After it ends, for the remainder of the battle, there will be a new attack to avoid. A yellow beam will mark a spot, then a stronger blast will follow suit. It is very easy to avoid. Since it tracks your last position before it locks on, you can be miles away before it fires. When Nightmare's energy is gone in this final round, his true core will show and begin to gain energy. DT and attack it with your most powerful attacks. The energy beams the core gives off can be
avoided, but now is not the time to be running away. If its core succeeds in restoring its health, you will have to fight it all over again. Use a Devil Star or an Untouchable if you need to finish the job off quickly.

Note:
You can gain an edge over Nightmare the first two times you fight it by lighting up the room before you trigger the Nightmare sequences. When you enter a room with 4 strange multicolored circles on the floor, that's a Nightmare room. Light the dials and then continue.