The Specter is a boss fought in Kingdom Hearts Chain of Memories. It is fought in two different stages.
Kingdom Hearts Chain of Memories
The Specter is summoned by Marluxia once Sora confronts him. Now fused with it, Marluxia has the Specter attack. However, Sora breaks the Specter's scythes and badly damages it. Marluxia decides to up the ante, enlarging the Specter, so he and Sora can face off one last time. Once Marluxia is defeated, the Specter vanishes into darkness with him.
The Specter at first appears as a giant Nobody with a large flower on top and two arms with scythes at the ends, as well as a billowing tattered cape and a long thin spire going down from its body. Its body shape is overall humanoid, aside from the lack of legs, substituted with the pointed spires that seem to comprise much of the overall Nobody aesthetic. Its cloak bears a hood, similarly to the Organization, although it is white-gray in color, and bears the Nobody insignia on its hood. Underneath, a head can be seen, although this is the only portion of the body that actually seems to bear physical structure, as the scythes are not actually connected to any appendages. From the flower atop its back, its master Marluxia blooms forth, almost like the pistil.
Once this form is defeated, the Specter seems to undergo a growth spurt and becomes a full-sized ship, reminiscent of other endgame battle formats such as the World of Chaos or Xemnas's Dragon Ship. The body is much more streamlined in this format, with the arms and general humanoid shape disappearing, even though the face can still be seen. The battle is fought atop the black cloak, where Marluxia, now immobile being held in place with thorns he seemingly uses for control, can be seen with the Specter in a new form, now having grown to massive size, possessing an enormous scythe of its own. In true Final Fantasy fashion, the Specter takes on angelic form, with two pairs of wings, one thin and black, and the other larger and white, bearing the shape of Marluxia's scythe. Pink-brown spiked tendrils, similar to the scarf of the Twilight Thorn, flow freely from the new "helmet" on top of its hood, which bears the vague shape of a skull. More such tendrils, black in color, billow from the sleeves.
Kingdom Hearts Chain of Memories
Marluxia fuses with the giant Spectre. Out of the two battles, this one is most likely the easiest due to the lack of sleights. The Second Form is fought largely in two stages. In the first stage, the objective is to destroy the scythe hands. The main attack it uses is a minor version of Deathscythe. It is possible to damage the main body in this stage but Marluxia is only vulnerable after a card break. Once the blades are destroyed, Marluxia takes on a different strategy. He uses floating flowers that shoot lasers, which can be easily destroyed simply by card breaking them. At regular intervals during the battle, he fires 2-5 slicer waves which can be dodged by jumping and heading to the fore/background. He also occasionally uses Blossom Shower, which can be dodged with a well-timed Dodge Roll.
After this, Marluxia hovers above Sora and slams the mechanical vehicle down, creating a shockwave that can be avoided by jumping at the right time. He does this three times in succession, and it is important to note that as his HP dwindles, the succession becomes faster and harder to dodge. After this attack Marluxia rears back and shoots across the screen. Card breaking this is important, as doing so puts Marluxia in his most vulnerable position. After breaking the attack, Sora is placed on the back of the mech and Marluxia sends out three more shot buds to protect him. Note that the longer Sora stays on, the harder it is to remain on the mech, as Sora is constantly forced backwards away from Marluxia. A good strategy is to use high level Cloud cards (around 7-9) in succession (one at a time, not in a sleight) while in front of Marluxia. Doing so deals a lot of damage in a short amount of time, and can also deal with the shot buds when card breaking them. Eventually, Sora is forced off, and the whole process begins again. This attack pattern repeats until Marluxia is defeated.
Kingdom Hearts Re:Chain of Memories
Marluxia (Second Form)
The battle in Kingdom Hearts Re:Chain of Memories is very similar to the battle in Kingdom Hearts Chain of Memories, although Marluxia spends much more time on the field and vulnerable to attack, the field is much larger, and Marluxia will often glide away from Sora. He keeps many of his attacks from Kingdom Hearts Chain of Memories, including the shot buds, the slicing waves, and the drop-down shockwave attack. Marluxia still uses only one card at a time, but his new attacks are more like sleights. The first of which is when he engulfs the field in darkness, preventing Sora from seeing his cards (except for the reload card), and the second is when he creates a massive Blossom Shower. The perspective in these attacks changes to Marluxia, which can make it difficult to dodge these attacks.
For the first part of the battle, the objective is the same as in Kingdom Hearts Chain of Memories: destroy the scythes. Marluxia will swing at Sora with his scythes in a manner similar to Deathscythe. This can be avoided by dodge rolling around the machine he rides. It is possible to attack Marluxia directly, but destroying the scythe blades first makes this easier. When destroyed, each of the blades releases a few HP orbs. When Marluxia's HP Gauge is reduced to three bars, any remaining scythe blades will be destroyed. After both are destroyed, Marluxia will rocket across the field in an attempt to ram into Sora, though this can easily be dodged with a well-timed Dodge Roll.
After that, he will summon three shot buds. These can be disposed of easily by simply card breaking them. Afterwards, he will create a rotating ring of five "X"-shaped flower waves and fire them at Sora. He does this faster as his HP gets lower. At regular intervals, he will use his sleight-like attacks: the mega Blossom Shower and the "dark void". As previously mentioned, the "dark void" attack turns all of Sora's cards completely black (with the exception of the reload card), preventing him from seeing them. He repeats this attack pattern until his HP almost reaches halfway, then he begins using the drop down shockwave attack. Marluxia does this three times in a row, then repeats the previous attack pattern. This continues until Marluxia is defeated.
Marluxia (Third Form)
Marluxia also has three regular attacks, the first of which are the shot buds from the second battle which have higher accuracy and card value than last time, the second attack is Marluxia's mega blossom shower which has not changed from the last battle, and the third attack is a blast of energy similar to Deathscythe.
This fight takes place as soon as Marluxia's second form is defeated, with no chances to modify the deck. So what will work well on both of Marluxia's final forms is before battling Marluxia, put in at least 12 attack cards, three or six of which are zeroes as Marluxia uses all of his attacks three times, except for the shot buds. Put in several Cure and Thunder cards and if possible put in some Cloud and Stop cards to use Cross Slash+. If using the aforementioned sleight put in more attack cards to use it more often and several item cards as well. Finally, put in the Vexen, Oogie Boogie, Jafar, Shadow, Trickmaster, Barrel Spider, and Air Soldier Enemy Cards. Guard Armor and Axel are also useful as Guard Armor allows Sora to attack Marluxia from a greater distance and Axel gives Sora a slightly better chance of escaping Omni Laser.
When the battle begins use either the Jafar or Trickmaster cards to make attack cards invincible or decrease the value of Marluxia's cards, respectively; these are pretty strategic as Marluxia uses high value cards at the start of the battle. Follow with the Shadow card to boost card values and try to activate Ars Arcanum, Blitz or Strike Raid to deal more damage to Marluxia; Ars Arcanum (three 1s) and Blitz (two 1s and a 9) become possible due to Shadow's effect. Marluxia's sleights have a maximum value of 24, so take advantage of Shadow to top this and deal some damage. After Shadow is used up, reload with an item card or let Marluxia use Gale of Severance to reload without interruption. Omni Laser is likely to follow, so use the Vexen card to increase chances of survival; if Sora is revived, follow up with Oogie Boogie to heal. Afterwards attack Marluxia like a normal enemy; use the Air Soldier card to dodge Marluxia's attacks and reload at the same time, and then use the Barrel spider card for faster reloading from here on out. Use any other tactics to finish off Marluxia if he still has HP left.