The Boss Guide from IGN (it tells you where to find the hidden characters)
For more assistance, check here.
From the first link:
Big Boo Battle
Completing this task will earn you the key for unlocking Luigi's door in the main castle. Enter the Big Boo's Haunt level (course five) and earn the very first star. Once you do, you can climb the steps in the main foyer of the haunted house. At the top of the stairs, turn right and go through the last door you see. Run past the bookcase to your left to find a small rise in the flooring (under a red box). Jump on that ledge and perform a wall kick or a triple jump to reach the hidden platform above you, and continue through the door to the attic. Jump through the painting of Luigi to start the Big Boo Battle.
The first part of this hidden area is a maze of rooms. To get through them, you'll have to proceed from room to room by going through the correct doors—if you go through a wrong door you'll be warped back to the beginning. First, head east, and then north. Head east again in the third room, and north again in the fourth room until you reach a hall with a long falling bridge. Run over the bridge and use the three checkered platforms to reach the top and drop into the hole.
Walk forward and stand in front of the big mirror. You'll see Luigi's reflection as Big Boo appears. Big Boo will go back and forth between being visible in your world and being visible in the reflection world. When Big Boo is only visible in the reflection, use the shadow on the ground near Mario as an indicator of where he is. Circle around Big Boo to get behind him and perform a dive attack to strike him from behind. Hit him three times with this attack and you'll earn the key that unlocks Luigi's room.
I'd actually type out my version of it, but I've forgotten over the past year or so. That's why IGN pwns, I guess.
