Kingdom Hearts: Birth by Sleep/Optional bosses

Armor of Eraqus (Final Mix)
The Armor of Eraqus uses most of Eraqus's techniques, but also imitates the protagonist's unique Command Styles and special abilities. Later in the battle, it can gain a magical seal on its back after ensnaring the protagonist; if the protagonist attacks the Armor in this phase, they will be stunned by the seal, leaving them wide open for the Armor to attack. Basic Keyblade strikes do minimal damage to the boss. The difficulty level of the boss fight is toned down in the HD 2.5 ReMIX version.

The Armor is particularly vulnerable to Shotlocks and Reprisals, and can also be damaged by reflecting its projectile attacks. Surge commands can also be used to deal damage while gaining range away from the Armor, while Mine commands will allow the player to lay traps without risking damage by getting too close to the Armor.

Iron Imprisoner I
This is the slowest of the Iron Imprisoner's four forms. In this form, the Iron Imprisoner is chained and trapped in a gibbet. It attacks mainly by swinging the gibbet at you or spinning around like a fiery tornado. It can also entrap you in a cage or erupt out of the ground like a volcano. The best way to start the battle is to use your Shotlock to shave away at its HP bar, then attack normally. It can occasionally shoot lasers from the bottom of its gibbet, dealing fire damage on impact, regardless of how much HP it has.

Overall, it is the easiest of the four forms. Merely dodge the lasers and attack. Alternatively, if you wish to stop any one of its attacks and keep it immobilized, cast Firaga on it repeatedly. While this will inevitably trigger the Firestorm Command Style (or Bladecharge), do not bother too much about attacking with Command Styles unless the player character's Command Style finisher is available for use, as he/she is invincible when performing finishes.

Iron Imprisoner II
In its second form, the Iron Imprisoner's arms are free, and it holds a massive hammer. It can also spin with the hammer outstretched, dealing fire damage on contact. It retains all of its attacks from its first form, only that it moves slightly faster. Simply dodge, attack and use Shotlocks or cast multiple Firagas on it when possible.

Iron Imprisoner III
In this form, the Iron Imprisoner's legs are now free of its gibbet. As a result, it is much faster, but it cannot shoot lasers. It retains its other moves so the strategy is the same. Shotlock commands are the most effective way to damage the Iron Imprisoner, and as a result the Damage Syphon ability is very useful. While it will pause periodically, the window of attack is very small, and it is safer to use the time to heal, use a Shotlock or cast multiple Firagas. As it HP dwindles, it's movement speed increases and it will unleash it's true strength and ferocity on the protagonist which will be a regular move for it's true, fourth and final form.

There are three safe windows of attack: after the Iron Imprisoner summons its cage, immediately after it emerges from the ground, or when it is very far from the player. The first two instances are safe for the player to use Shotlocks; the last instance requires multiple Firagas to subdue the Iron Imprisoner.

Evading the cage is difficult unless the player is executing a finishing command or a Shotlock. If timed correctly, one can avoid the cage by attacking the Imprisoner with a combo.

There is a foolproof way of dodging the cage and preventing it from imprisoning the player: when the cage is still in the ground following the player, dodge in the opposite way in which the cage is going. That way, the minute the cage meets the player, it will rise from beneath the ground and no longer chase the player around. The player, having dodged, will be free to move. This move, however, is not recommended when the Iron Imprisoner is down to its last two bars of health, due to its more vigorous attacks.

Once it unleashes it's true strength and ferocity due to less HP, it is mandatory to block and remain in the stance until he finishes the attack. Keep moving as it's movement speed increases.

Iron Imprisoner IV
In its final and most powerful form, the last pieces of miscellaneous binding have been removed, enabling its full moveset. The cage it traps you with is now on fire and will damage you on contact. Its hammer also doubles as a torch, inflicting more damage than in its previous form.

It also has an attack where it sucks you in close before releasing a large explosion. It is wise to keep a further distance away from the boss. If the protagonist is sucked in, it should not be troublesome as the explosion can be blocked. During another new attack, it will possess the room, turning it red and causing the tiles to fly off and attack. The strategy for the third form is also effective here, and the boss can still be stunned by a successful Firaga or Sonic Blade. However, because of the Iron Imprisoner IV's increased strength and ferocity, the player should be more cautious. When the boss possesses the stage, the player can either choose to stay in one place and block constantly to defend against the flying debris, or simply Dodge Roll, Cartwheel, or Slide around or run to evade damage. It would also be wise to equip a Vanish command, which can be bought in the medal store, as when the character is invisible, the Iron Imprisoner mostly avoids them, giving you a chance to heal or use a shotlock.

Mimic Master
The Mimic Master will appear as the final boss in the Dead Ringer round, one named specifically for it, four waves of lesser Unversed having to be defeated before it can be faced. This boss can be quite challenging to the unprepared or reckless player, so be sure that the protagonist is at a relatively high level and has the strongest abilities as possible equipped. Shotlocks, Command Styles, and D-Links are also available for use. Depending on the character the player chooses to battle as, a different strategy may need to be used than one that can be used for another character. Considering the player will have a rather high attack power and friends may be present to aid in the fighting process, these are advantages that just may turn the tide of the battle and should not be taken for granted.

At the start of the battle, the Mimic Master will fly around the arena, its book-like weapon engulfed in damaging flames. Use Block or Barrier (depending on the player's character) to avoid the attack. Remain in constant motion if the Unversed boss is performing this tactic out of range to avoid it catching up to the player character. When the Mimic Master settles down, the player can then proceed to bombard it with long-ranged attacks such as Barrier Surge or even standard physical combos (if one is careful) with a powerful Keyblade such as the Ultima Weapon. The player should be careful if fighting it at close range, as it can attack players by smacking them with its book. The player should then flee if a counterattack seems inevitable and set a trap for the boss such as a Mine Square attack for it to run into and receive damage from as it attacks. This will, in turn, interrupt its dangerous combos. If the Mimic Master vanishes for a moment and its book levitates around the battlefield, jump and use an aerial movement ability such as Double Flight (Aqua), Glide and Superglide (Ventus), or Air Slide to escape the flurry of pages it sends out as it reappears to attack the player.

If pages from its book start to levitate around the Mimic Master, it will shoot long-ranged lasers at the player as it spins slowly around the battlefield. These laser beams can Blind the player if they aren't careful, so jump and use an aerial movement ability each time the lasers are about to hit in order to avoid them as best as possible. While the Mimic Master will only reuse previously seen attacks for the majority of the remaining battle, it has one last, deadly technique it will use as its HP dwindles. First, the Unversed boss will shine with a dark aura. It will then vanish, its book starting to glow. It will then rush at the player, the boss reappearing as a splitting image of the player's character. While in this form, the Mimic Master's HP vanishes, replacing it with a format similar to the player's. It will also copy the player's nickname with mixed letters as a confusion technique. To revert the Unversed to its true form, merely destroy the book pages it scatters across the battlefield. The Mimic Master will then repeat its cycle of attacks from the beginning until its eventual defeat.

Monstro (Final Mix)
The battle begins inside Monstro's stomach, where the protagonist will be faced with a squad of Unversed that must be defeated in order to get Monstro to sneeze the player above the surface of the ocean.

Once outside, a three-minute timer will start counting down, in which the protagonist must deal as much damage as possible to Monstro before it runs out, though it can be tricky due to Monstro's mobility and the scarce attacking ground. Monstro will occasionally slam his tail on the arena, dive across and even try inhaling the player to sneeze them out and deal damage. The main strategy of the battle is to hit Monstro as many times as possible with the explosive barrels while he is swimming slowly. Once hit enough times, Monstro will go dizzy and leave his tongue hanging out, a sign for the protagonist to get close and use a Shotlock, possibly twice if quick enough. Occasionally, Monstro will slam his upper body onto a small island and briefly leave himself open to melee attacks.

If the timer runs out, Monstro will automatically swallow the protagonist again, facing them with another squad of Unversed; each time the protagonist returns, the amount of stomach acid increases, leaving less area to fight. The final round occurs after the fourth time the protagonist escapes, and will have no timer.

A high-risk but fast method of winning is to equip five Mine-type commands, one Mega Flare command, and one Group Cure-type command (to increase medal payout). The player should run up to the center of Monstro's stomach and quickly use Mega Flare to destroy the Unversed and force themselves out onto the islands; the player should jump to the largest patch, use their Group Cure command, and may opt to use a Mine command should Monstro fly past. Eventually, Monstro will slam himself onto land, and at this point the player should expend their remaining Mines in the hopes of dealing as much damage as possible; with some luck, it is entirely possible to end the battle on the second round.

No Heart (Final Mix)
No Heart is one of the strongest secret bosses in the game, rivaled only by Young Xehanort. His attacks are strong and can deal heavy damage. In terms of preparation, the character should be at a high level, have a strong Keyblade, max HP, and lots of Cure and Potion commands, as well as have the Second Chance and Once More abilities equipped.

In the first phase of the battle, No Heart remains stationary, protected by a large barrier of dark energy. To disable the barrier, the character must attack No Heart's Keyblade, which is constantly floating around. In terms of offense, No Heart will summon a swarm of flying Keyblades from the Keyblade Graveyard that can be blocked, or even destroyed by commands such as Raging Storm, to attack the character and sometimes use an advanced Corridor of Darkness that absorbs and traps characters on the barrier (Circle must be pressed repeatedly to break free), and may use Dark Volley. No Heart's Keyblade is vulnerable to any attack. After dispelling the barrier, the fight becomes more serious as No Heart rises from the throne, removes his cape and challenges the character directly.

This is the harder phase of the battle. The boss possesses an arsenal of potent dark-based attacks which can deal large, and possibly fatal damage. No Heart mainly uses Sonic Impact, darting around the arena to attack the character. The boss can utilize Dark Volley to strike from a distance, as well as summon flying Keyblades to attack. The boss can create waves of dark energy by swinging his Keyblade. He may also leap into the air and slam the ground to create a large shockwave. No Heart can use Reversal to maneuver around the character to unleash more Keyblade strikes, and may completely block attacks with his Keyblade. No Heart also can create large pools of dark energy that will either slow down or speed up anything that is inside, and may even bind the characters in place with no means to move aside. The armor can also alter the shape of his Keyblade for strong attacks (a similar fighting style to the Lingering Will) such as turning it into a claw to grab characters (Circle must be pressed repeatedly to break his grip), a spinning duel edge blade, a gun that can shoot powerful shots, morphing it around his leg for powerful kicks, and turning it into a wing to fly. From the air, he will either shoot more dark energy blasts, summon more flying Keyblades to attack, dive at the hero repeatedly, or use his most powerful attack: A large explosion of dark energy that will instantly drain all HP, Commands, the focus gauge, and may leave the player character stunned if characters are in the vicinity. He may also telekinetically command his Keyblade to attack characters directly and to absorb any damage directed to him.

Heal Strike, Renewal Block or Barrier, and a Command Deck containing Thunder Surges alternating with Cure spells will lead to an easy victory. Once More and Second Chance should be equipped as well. Thunder Surge is particularly useful compared to other techniques because it can lead into the powerful Rhythm Mixer Command Style. Victory against No Heart depends on careful timing and conscientious healing.

In the HD 2.5 ReMIX version, like with Young Xehanort, the difficulty of No Heart's battle appears to have been toned down. While he is still as dangerous as in the original version due to his increased evasion and speed in the HD Remix, he is now vulnerable to Shotlock attacks like Aqua's Lightbloom, which take away a good portion of his health.

The reward for defeating No Heart is the Royal Radiance Keyblade.

Vanitas Remnant
The Vanitas Remnant can be fought in the Badlands area of the Keyblade Graveyard after the player clears the Final Episode. Defeating the Vanitas Remnant in the Japanese version of Kingdom Hearts Birth by Sleep earns the player a Secret Gem. In the North American, PAL, and Final Mix versions of the game, defeating this boss unlocks the Void Gear and allows the Unknown to be challenged.

Terra, Ventus, and Aqua cannot use D-Links against the Vanitas Remnant. However, they still have access to Command Styles and Shotlocks. While the Vanitas Remnant will avoid most of the damage Shotlocks cause, the brief invulnerability these techniques grant makes them useful for enduring the boss's attacks. For maximum efficiency, the protagonists' equipped Shotlocks should be at their highest levels.

Depending on the approach to this battle the player wishes to take, their Keyblade of choice will vary. Keyblades with a high Strength stat are best used if one intends to fight the Vanitas Remnant using Aerial Slam. Magic-oriented Keyblades, meanwhile, are recommended if one plans to use Tornado or Seeker Mine.

Terra, Ventus, and Aqua should rely on the most basic forms of their respective dodging techniques when fighting the Vanitas Remnant, as their more offensive versions do not grant the same windows of invincibility. The protagonists should also use Potions or techniques such as Heal Strike, Renewal Block, and Renewal Barrier to recover HP, for Cure and Friend Cure spells allow the boss to also rejuvenate itself.

To compensate for his lack of speed and poor dodging ability, Terra should only use the fastest of attacks when facing the Vanitas Remnant. It is also recommended that he enter the battle against this foe with two sets of Potions, a high-level version of Mine Square or Mine Shield, all three available HP Boosts, and the Second Chance and Once More abilities equipped. To deal as much damage to the Vanitas Remnant as possible, Terra should use the Chaos Ripper Keyblade, as well as the Demolition Finish command.

When dealing with the Vanitas Remnant's Clone Burst attack, Terra should use Slide to avoid taking damage. He should not use Sonic Impact, as this technique lacks Slide's temporary invincibility. Terra can also use Keyblade strikes and fast-paced, long-range attacks like Thunder Surge to delay Clone Burst's execution. When the Vanitas Remnant uses Dark Fang, it is recommended that Terra use Slide to quickly reach the boss and then use Mine Shield or Mine Square; it is likely the Vanitas Remnant will walk into the mines and take damage in its attempt to continue attacking. If the Vanitas Remnant uses Dark Cannon, Terra can use Ars Solum to deal heavy damage. Alternatively, he can consume a Vanilla Glitz to gain early access to the Critical Impact Command Style; this will greatly increase the damage of his standard attacks, which can be used until the boss's Shotlock ends.

When fighting the Vanitas Remnant, Ventus should use Dodge Roll to avoid taking damage. He can use Superglide to quickly dodge the boss's ranged attacks. It is recommended that Ventus have at least two sets of Potions equipped during this battle, as well as the Second Chance and Once More abilities. Tornado and Mine Square should be his main source of offense, and he should use Renewal Block to defend against the Vanitas Remnant while recovering much-needed HP.

To avoid Dark Fang, Ventus should use Dodge Roll. The temporary invincibility the technique grants him will prevent him from being inflicted with Blind. Ventus's Stratosphere Finish command also bestows brief invulnerability; it should be used in place of Command Styles. Ventus's Tornado spell is most effective at the conclusion of the Vanitas Remnant's Dark Thundaga, Clone Burst, and X-Discharge attacks. Aqua

When Aqua faces the Vanitas Remnant, she should have Seeker Mine and all three available HP Boosts equipped. Her Keyblade of choice should either be the Stormfall or Ultima Weapon Keyblade, as both are proficient in Strength and Magic. To avoid the Vanitas Remnant's attacks, Aqua should chain multiple Cartwheels together.

In addition to Seeker Mine, Aqua should have three sets of Potions and four sets of Fabracadabras equipped. After three quarters of the Vanitas Remnant's HP has been depleted, it is recommended that Aqua start using Fabracadabras to gain early access to the Spellweaver Command Style, which she can use until the boss is defeated. While not particularly powerful, Spellweaver grants Aqua invulnerability, has great range, and can hit multiple times.

A strategy that involves the consumption of fewer items relies on a combination of Seeker Mine and Teleport Spike as its main source of offense. Aqua should use this command until three quarters of the Vanitas Remnant's HP has been depleted. She should then use consecutive Counter Blasts to fill her Command Gauge until Teleport Spike becomes available. It is recommended that Aqua utilize this Finish command until the Vanitas Remnant is defeated to gain temporary invulnerability and the ability to strike multiple times.

A glitch in the Vanitas Remnant's A.I. allows for an easy victory against it. Simply lure the Vanitas Remnant to the large rock formation in the corner of the battlefield and retreat to the rock's opposite side. The Vanitas Remnant will be left unable to attack. Terra, Ventus, and Aqua can effortlessly defeat the boss with a Command Deck comprised entirely of Strike Raid commands.

Unknown
Young Xehanort is an optional boss in the international and Final Mix versions. His boss battle is unlocked when the protagonist returns to the Land of Departure after defeating Vanitas Remnant, on a save file in which it is still accessible. Despite this odd situation, his scene is within canon, and is considered to occur at the save file's point in the story. Defeating Xehanort will grant a unique Keyblade named "No Name", but he can be fought again by leaving Land of Departure and then returning.

Young Xehanort is a powerful boss and one of the strongest enemies in the game. His attacks are varied and hard to predict, similar to those of the Lingering Will. Not only is he immune to all status effects, but he has a great deal of HP as well. The boss is very swift and can inflict massive damage to the protagonist, quickly knocking them out unless they have Second Chance or Once More equipped. Xehanort is also adept in defense; he can use Renewal Barrier if attacked too much, and he sometimes rewinds time after taking damage, allowing him to recover some of his lost HP.

As Xehanort's HP dwindles, he goes even further on the offensive. He will often start one of his long-duration techniques, then continue to attack with different techniques while the first one is playing out. Xehanort will also clone himself, creating four copies that can attack and move independently of the true Xehanort. However, the clones can dissipate after being hit once, so try to use a multi-hitting attack to clear the clones. His most frustrating tactic is to snare the protagonist with his Collision Magnet, then immediately following it up with another attack and a game over (although there are very rare occasions in which the protagonist can escape before Xehanort strings together another attack). Collision Magnet can be avoided easily by simply dashing towards Xehanort when he jumps in the air, and also seems to be canceled if Xehanort pulls the protagonist up against a wall.

After half of his heath is gone, Young Xehanort will begin to turn invisible. While he cannot be locked on, he does leave a faint shimmer and can be "visible" through his Ethereal Blades, so he can still take damage. He still uses his old abilities, but since he is invisible, it is harder to see when he uses them. However, he also gains new abilities, such as firing a beam that inflicts Doom and forces mashing X to escape, and casting Mega Flare and Meteor to inflict massive damage.

Xehanort will attack whenever an opening to do so presents itself, but it is easy to heal safely by simply dashing far away from him first with either Slide, Dodge Roll, or Cartwheel. It is also somewhat easy to guard against his attacks by mashing Square, so the protagonist can also heal by using Renewal Block or Renewal Barrier. Because Xehanort leaves little time for the protagonist to act, it is unwise to use any advanced or ultimate commands, as they take too long to fire and reload. Shotlocks are also good options, but Xehanort likes to evade the targeting reticule, and can often evade damage midway through by using its Renewal Block, which will also heal him. If the protagonist decides to rely on Shotlocks, they should stock many Elixirs in order to make sure their Focus Gauge never runs out. D-Links can also work, especially Goofy's for Ventus and Aqua due to the Goofy Spin finisher allowing free movement and rapid blows. However, Xehanort may turn invisible in response, so it may not be ideal to use them. Finally, since Xehanort leaves little time for Keyblade combos, the protagonist's equipped Keyblade and finisher are not important, but the Keyblade equipped should lean towards the Strength stat.

The safest and simplest way to defeat Young Xehanort is to simply use a deck of 2-4 Cure-type commands, and fill the rest with Fire/Thunder Surges. Thunder Surge is generally better because it grants access to the Thunderbolt Command Style, and Thunderbolt's finisher, being an area-of-effect attack, is more effective at striking Young Xehanort than Firestorm's. A good way to arrange the Command Deck is to have Thunder Surge, Cure, Thunder Surge, Cure and so on, so that both commands are easily accessible and it also helps with not needing to cycle through commands that are reloading. With this deck, the protagonist can basically use Thunder Surge over and over, even when Xehanort is invisible, damaging Xehanort if they hit him and often knocking him out of his combos. The protagonist should make sure to use Thunder Surge so that they go through Xehanort and winds up a short distance past him, rather than next to him, but should dash away instead if Xehanort is right next to the protagonist, as Thunder Surge has a tiny but noticeable start-up lag. Thunder Surge can even be used as a pseudo-dash in order to avoid Xehanort's techniques, like his Collision Magnet. If the protagonist is ever hit by an attack, they should immediately start dashing away, normally, and heal with Cure as soon as they are clear from Xehanort. If playing as Ventus, it's a good idea to stock up on Double Crunch Ice Cream to activate Wingblade, which allows him to hit Xehanort with a combo of Keyblade attacks that he can't easily break out of. Even after setting up a working deck and overall strategy, victory against Xehanort relies primarily on luck, due to his penchant for using unavoidable, instant-death combos.

In the Final Mix versions, the difficulty of the boss fight is slightly toned down. The protagonist can now escape after being ensnared by the Collision Magnet attack by Air Sliding before the second attack, therefore avoiding the lethal combos that were inescapable in the original version. In addition, Thunder Surge is even more useful, as it can easily lead into the powerful Rhythm Mixer Command Style.