Instructions
Enter the room and wait for the animated sequence to stop. The first two Dementor-shaped Boggarts will be hovering in the room. Place the Wii remote target over one Dementor and hit the "A" button to zap it, then hit it with the Patronus charm by waving the remote and using the "Expecto Patronum" spell. Repeat for the second Dementor; both will disappear.
Head to the left side of the room; you will see a blue pixie. Zap the pixie and it will release the Dementor, then use your Patronus to kill the Dementor.
Move to the right side of the room, where you will see vines; there is a Dementor hiding amongst them. Use Patronus on the Dementor.
Open the storage cabinet next to the room's exit. Assemble the movie projector inside, it will project an image of a Dementor. Use Patronus on this Dementor. The exit door will open when you defeat the final Dementor; go out the door into the hallway to leave the room.