How do I get the last star in whomps fortress?

How you unlock luigi is you got to go to big boo's haunt and go near to the top and you will see a pic of luigi and go in that then you will here a boo laugh follow that and go up the platform and face big boo by ground pounding him and do that 3 times and that is how you unlock luigi

First you need 15 stars and Mario. Then go to the court yard and look for a ghost with a cage in it. Then ground pound that ghost and walk up to the cage. Pick any level on big boos haunt. Then go in the mansion and either take the stairs or triple to get to the second floor. Then you go the east door on the second floor. Then when you're in that room do a wall jump on the east wall and you will be on the third floor and there will be a picture of luigi. Then jump in it(who would not!). For this next part I strongly reccomend using your mini map for the directions I give you. Now you are in a merry-go-round with 4 doors. Go through the right door thenthe upper door then the left door then the then the upper door remember to use your mini map! Then run across the next room and then ride the green plat forms up then jump into a tiny hole. Now you are in a room with a BIG mirror walk up to the mirror and your reflection will change into Luigi then a ghost will apear in the mirror then use your reflection to ground pound the ghost 3 times after you defeat the ghost luigi's key will pop out of the miiror and that is how you get luigi's key.