In its blob form, you cannot damage it in any way. However, it can
attack you with a myrid of strange but powerful hits. The most
important of its attacks will be to clamp you up in a shell of sorts,
and send you into the Nightmare World.
Other attacks include:
1. Sending a couple of worms to curl around Dante's body. They will
explode into ice shards after a while.
Rolling Blaze to drop them off, and get away from them as well.
2. Shoot dark arrows at you.
Run. Run. Run.
In the Nightmare world you're faced with a couple of Skulls, plus one
big one. Nothing too hard, a single shotgun blast(s for the big one)
will break them up into little pieces. When you're clear of them
though, you'll be facing an old enemy: Nightmare Phantom.
Use the same methods that you used on Phantom 3 on it. Its attacks are
exactly the same. Moreover, it has a lot less life than Phantom 3.
Lastly, defeating it will grant you one whole green orb for your
troubles.
Once you've defeated Nightmare Phantom, you'll come back to the real
world...taking away 20% of Nightmare's remaining health. The rest of it
you'll have to deal with yourself.
Head for ANY of the Wall Symbols and whack on them until they max with
yellow symbols (Note that if you hit them BEFORE getting caught in the
Nightmare World the symbols will flash white instead). This will
lighten up the room, and reveal Nightmare's main body. The main body
will reveal itself AS LONG AS the Wall Symbols are lighted up. Oh, and
make sure you're equipped with Ifrit while you're at it.
Now then, on to the main body's attacks:
1. It will shoot hundreds of bullets at you.
This is ALWAYS the first attack, and it's easy. Start in one direction,
strafe AROUND it, while shooting your handguns. This will not damage
it, but up your DT gauge, which you'll need. A LOT.
2. It will suddenly turn to the side, and stick out something at you in
3 successive hits.
This one needs a little work. If one hits you, you'll be hit by the
second as well. The timing is crucial because you only have a split
second to SEE it turn to you. IMMEDIATELY jump around it (Rolling Blaze
is a MUST, or else you'll get hit), countering the spike, and stunning
Nightmare to reveal its core for you to attack.
3. It will charge up a huge blast, while turning slowly to face you
while revealing its core.
Simple. Jump up onto its back and whack the core.
4. The core will open at its back, and shoot out 10 homing missiles at
you.
Again timing is paramount. Mistime your jumps, and expect to get hit 10
times. Jump towards the missiles and with Rolling Blaze activated,
it'll deflect the missiles BACK to hit the core.
Speaking of the Core, that's the only part of Nightmare that you'll be
able to damage. It'll reveal its core for a few seconds after attack 2
or when you counter attack 2, during attack 3 and 4.
What to do? If you've successfully countered attack 2, immediately get
near and use Inferno, and watch its health get eaten up. If you don't
have Inferno at this time, use Air Raid instead. Note that after attack
3 and 4 the core will stay vulnerable for a couple of seconds, but at
that point of time it'll also be sucking away your valuable Devil
Trigger gauge.
After it's done with attacks 2,3 or 4, it'll go back to attack 1 again.
Repeat the process to slowly but surely defeat it.