Go in the house as mario. Head straight, then left. Go in the blank door, NOT the 3 star one. Next, chase the boo into another door. This is the garden, but don't be fooled. Head left, and look at big boo. Then try to sneak up on him and punch him. Once you've done that, jump in the cage. Welcome to big boo's haunt! Now, to get Luigi in this mansion:
Head straight and go into the mansion. Run foward and triple jump onto the second floor. From there, head to the farthest right door. Grab the power flower, and float up to the secret door. Ground pound there, and go through the door. Head right, and go in the Luigi painting! Head to the right door. Then take the left. Take the right one again, and then take the elevator thingys to the farthest left one. So take three of those things, and run foward all the way to the painting of boo. then take the lifts, and head in the chimney hole. Run up to the mirror, and watch as Big boo comes. Now sneak up on him using his shadow and the mirror. Punch a lot of times, and the luigi key is yours.