Mission 28: Collect hearts
Roxas and Xion continue to work together to collect hearts, hoping to compensate for Xion's lack of ability to use the Keyblade. While at Beast's Castle, they overhear the servants talk about the mysterious master and guest.
Another straightforward, gather hearts mission. This one starts in Entrance Hall Upper Level, which you'll clear out first. There are four Bad Dogs there, two in the northern curve near the stained glass window and two in southern cross hall with the crates. There are also a few Shadows lurking, but as always, they will not count toward your gauge total. Head down and out the southern entrance to the Courtyard. In the Courtyard are a couple of chests, an Ordeal Badge, and two Sergeants to add to the Mission Gauge.
Go into the castle, and after a short cutscene, Cogsworth will be patrolling the Entrance Hall. You want to stay out of his line of sight, but he makes it tough. He moves in a rough circle in the center of the hall, but is prone to abruptly turning and searching the opposite direction. There are two Soldier Heartless to fight in here, each between the doors on the eastern and western walls near the chests. They will appear just out of Cogsworth's range, but be careful when you battle them that you do not push them in his direction! Head for the north end of the hall and head up the stairs to the East Wing.
The East Wing holds the last of the Heartless: two Bad Dogs and a Snapper Dog. If you've gotten the other Heartless, defeating these three will push your Mission Gauge past the minimum requirement and into bonus territory. Once you've beaten them, don't forget to run to the end of the East Hall and snag the Unity Badge. All you need to do now is head back to the Upper Level the way you came, only having to worry about Cogsworth on the way back.
With so few restrictions, you can complete this challenge fairly easily. One viable strategy is to bring a number of magic casts, as well as magic-restoring items, and weaken or take out your enemies from a distance, coming in close only for final blows. Fire is a good choice of magic, as all of the Emblem Heartless in this mission are weak against it.