I've just beat the second fight with bowser and I want to know how to get..

He's being held captive by King Boo (which I can't beat) at Big Boo's Haunt. Here's how to get him. You'll need Mario. Ok, in the lobby of the castle, go on either brown door with no star on it surrounding the staircase. Scare the Big Boo away. Follow it into the courtyard. Ground-stomp/pound every boo that gets in your way. They all will have red coins, but since your so far in the game, I'm not sure if the red coins are there anymore. You might have to talk with the Toad at the portal to Tick-Tock-Clock. I don't know if that's the portal, but just know that it's not the one to Dire Dire Docks. Do a ground-stomp/pound on Big Boo. He'll leave behind a cylinder-like thing. Run into it to transport to Big Boo's Haunt. Beat the mission "Go on a Ghost Hunt", and defeat Big Boo after that in the lobby of the Haunt. You'll gain access to the 2nd floor, and access to beating King Boo. Go up the 3 giant stairs to the 2nd floor. Head to the door on your right, then get the Power Flower in the box of that room. Use Mario's Float Power to go to the platform above. Enter the door there, and you'll find two things, a portrait of Luigi, and the double doors lead to Big Boo's Balcony. Since, you want to know how to get to Luigi, jump through the Luigi painting. There are tons of doors and wheels there. I forgot how to get ot King Boo that way. Just keep wandering around until no more wheels, and you see a Mr. I. I forgot which door led to King Boo, so check all the doors there. The one without a wheel leads to King Boo. Go follow the path and be careful of it, you can fall off, so you have to run to the other side. Go on the platforms, upward, and go ahead. Fall into the hole followed by the red carpet. Run near the mirror inside, and past the carpet. Just stay there, and your reflection will turn into Luigi. King Boo will appear, and he'll say that when you only see his shadow, the only way to see him is looking at your reflection, or rather Luigi. He'll copy everything you do as the reflection. You'll see King Boo's reflection too. The only way to attack him is by punching him from behind. Kill Big Boo, and he'll give you the key to Luigi's room.

Everthing the guy said about how to get there is right but there is another way to defeat him and thats instead of landing the backflip hit R when you are above him to do a ground poundand it will hurt him and you don't have to worry about him turning around cause he wont move, you have to hit him three times with this and he'll give you a key