Armored Ventus Nightmare
The Armored Ventus Nightmare is the final boss of Kingdom Hearts 3D: Dream Drop Distance.
 Kingdom Hearts 3D: Dream Drop Distance
When Riku travels to Sora's Dive to the Heart, the Nightmare Armor appears out of the darkness and engages him in battle. Upon the Armor's defeat, the darkness covering Sora and his Keyblade cracks and fades away, leaving a comatose Sora behind as he falls into a pool of darkness.
The Armored Ventus Nightmare is Sora clad in Ventus's Keyblade Armor, which is now black in coloration. The Nightmare Dream Eater emblem is emblazoned on the visor. The boss wields Sora's Kingdom Key, which is also colored black, with a reverse grip.
The Armored Ventus Nightmare has less health the young Xehanort but often melts into the ground, making locking on challenging. The player can follow its path by the bubbles it leaves in the floor, but this is tricky, and may prove too much of a distraction. At these points, it may be better to stop moving, and be prepared to guard until you've either locked back on or spotted it. In general, the Nightmare mimics Ventus's style, making use of fast combos, counterattacks, and reprisals. Because of this, Dark Barrier and Counter Aura are recommended. As for Dream Eaters, it may help to have at least one with a healing ability, but other than that, it does not particularly matter. Since very few attack commands will be effective, you may consider bulking up on Cure spells and items, or on projectiles. Ironically, the air is generally a blind spot for the Nightmare, so if you have a command that can be quickly executed from the safety of a double jump, that may also help.
When it turns the background white, it may follow it up with one of two abilities. It will either generate four orbs of darkness that bounce around the arena, seeking out Riku, or attack by moving towards Riku and using combos. In the former case, the player can only dodge. This is best achieved by using Flowmotion to jump up the sides of the arena, where the orbs will not follow. In the latter case, the Nightmare can be damaged. Again, Counter Aura will prove effective here. In either case, timing is critical to defend yourself, and it will eventually melt back into the ground, returning the arena to normal.
If it leaps into the air, resist the urge to attack it, as it is about to fire orbs of darkness into the ground. Even if you avoid the initial attack, it will split apart and travel across the floor of the arena, and even if you avoid both, it will often quickly follow up with one of the white background attacks. The Nightmare may spin-dive into Riku, or melt into the ground and reappear behind him to attack. These are all best handled with Dark Barrier and Counter Aura. A trickier ability it has is when it gives the arena a purple tint and causes darkness to swirl around it, mimicking an aero or tornado spell. Since this all happens near-simultaneously, and since it often uses it when being damaged many times in rapid succession, the attack is very difficult to avoid. If hit, the attack will do moderate damage and hurl Riku backwards, negating whatever action he happens to be performing.
While Counter Aura is crucial in this fight, it does little damage, so relying on it to finish the boss may be tedious. It is generally safe to unleash a short combo whenever the Nightmare emerges from the ground and does not immediately launch into another attack. It will quickly attempt to counter, in which case Dark Barrier may be utilized. It may be tempting to use multi-hit attacks like Dark Aura or Dark Splicer on it, however, this does not work as well as it did on Young Xehanort. The Nightmare can change tactics quickly, opening you up to damage and/or knocking you out of the attack early, effectively wasting the command. It will often use "dark aero" to accomplish this.
In any case, once its HP is brought down to 1, the battle will shift. If the background is currently white, you must weather the attack until it returns the arena to normal, otherwise it will head straight for the center. Once there, a cinematic will play in which the Nightmare uses a "Shotlock" style ability on Riku, who counters by using a special Dual Link. The results are the same no matter which Dream Eaters Riku is using: He will begin glowing, and all of his commands will be locked, with his attack replaced by "Nightmare Clash." The Nightmare will take on a new, relatively small HP gauge, which must also be brought down to 1 to finish the battle. It will cloak itself in a purple light and perform several rushing attacks, which can easily whittle down Riku's HP even at high levels. Using Nightmare Clash will cause Riku to charge the Nightmare and damage it, also nullifying damage done to him if they collide. You can attempt to time the strikes, but it may be best just to max out Riku's HP and obtain Once More, which aren't very hard to do in this game.
Should you fail, the Nightmare will trigger a purple energy attack that is near impossible to avoid and cannot be interrupted. Also, your commands will still be altered, leaving no way to heal Riku. Mechanically, this is considered a separate combo, so Riku will most likely not survive if hit. Should you succeed, the Nightmare will be stunned, and the Finish command will destroy it for good. This command cannot miss, so perform it as quickly as possible, otherwise the Nightmare will regain consciousness, and you'll be forced to repeat this sequence with diminished HP.