Mission 50: Vanquish the Heartless threat
An unidentified giant Heartless has surfaced in Beast's Castle. Find it and eliminate it at once.
Roxas is sent to Beast's Castle to eliminate yet another Heartless boss. After defeating the Heartless threat, Roxas learns of love - through the Beast's unyielding protection for Belle. As Roxas watches the Beast and Belle talk, Xaldin appears and tells Roxas that love is a weakness.
You start in the Ballroom, work your way around the room defeating Heartless and opening the two chests. Exit the Ballroom to reach the Entrance Hall, immediately jump up to reach the Ordeal Blazon floating above the steps. Continue defeating Heartless and unlocking chests. If you prefer to save your strength and magic for the upcoming boss battle, you may skip defeating the small-fry Heartless in the Ballroom and Entrance Hall. Leave the Entrance Hall when you're finished and go to the Courtyard.
Here you will find the Unity Badge to the right of the entrance (near the secret entrance). Proceed towards the gates to watch a short cutscene, then continue on to the Bridge, where you will find the Heartless threat.
Once the Infernal Engine is defeated, you can continue to the end of the Bridge and get the Ordeal Badge. With the boss defeated and all treasures obtained, its time to RTC. Return back to the Courtyard to see a cutscene between the Beast and Belle, showing their love for each other. After the cutscene is over, the mission will end automatically.
This mission will be easier to complete after finishing Story Mode. You'll want to have Glide and Haste equipped, as well as Thunder and Thundara magic. Skip all Heartless battles inside the castle, and hurry to the Bridge. Use your magic to attack both the Infernal Engine and the lesser Heartless on top. Fight the boss as efficiently as you can, attacking the battering ram when it is exposed, and deflecting its cannon shots. Alternatively, using close range attacks and deliberately getting hit to use Limit Breaks is also useful as long as you have a high level with a Keyblade with high attack power, and the Auto-Life, Final Limit and Limit Boost panels equipped. If necessary, use a Limit Recharge. Since you do not have to fully backtrack to your original entry point after defeating Infernal Engine, this can also help save time.
This mission is also easier after completing Story Mode. Plenty of Thunder, Thundara, and/or Thundaga casts so you can attack from a safe distance. Also be sure to block and deflect the boss's attacks whenever you can. You can also deflect cannon shots with Guard to defeat all but one of the Heartless on top (to lessen the arrows that rain from above and prevent the other Soldiers from ever regenerating) and then keep using close range attacks to help expose the battering ram to make it easier to damage it and getting a good distance away before Infernal Engine can attack with it. However this strategy requires a lot of training to get used to it, and may require multiple tries to use it successfully, but also makes it possible to collect the 3 Challenge Sigils as early as possible as long as the right Keyblade is used without interfering with mid-air strikes.