Mission 50: Vanquish the Heartless threat

Mission 50 for Kingdom Hearts 358/2 Days sends Roxas to Beast's Castle to eliminate another Heartless.

Regular Mission
Vanquish the Heartless Threat.

Story Summary
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.

Mission Walkthrough
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.

Main article: Infernal Engine

Challenge Mission
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.

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.

Video
Video walkthrough here.