I don't know about you, but for me the easiest way is to turn your back to him, do a back flip and a ground pound.
Go to the top floor and go in the right most door. Then be Mario and use the , box to float up to the platform. Go in past the Luigi picture and you should see some doors. Go thru them.
You can't talk to boos because they are your enemies. The only things you can talk to are Pink Bob-ombs, and Toads.
kill i think 8 boos
its in a picture in Boos mansion in one room with a ? block use Mario then float up to the next room and it will be in there.
kill all the other boos by ground pounding on their heads
you cant Yoshi cant kill ghosts
you defeat all of the boos (ghosts) and the star will appear
its in boos mansion go in the ROOM with the ? block with Mario then float up to the next room and the painting will be in there to rescue luigi
Go to the entrance of the castle where the light is shining down and look up. You will be teleported to an area high above the clouds. There you will find the switch that turns all "!" blocks solid. Then go to Big Boos Haunt and go to the library. There you will find a "!" block. Make sure you are playing as Mario. Grab the Power Flower and inflate into a balloon. Float up to the secret balcony and go through the door. There you will find a picture of Luigi. Jump into it and beat King Boo to unlock Luigi.
level 12
There is no Bowser in Board Bowser's sub in Super Mario 64. But, by any chance, are you talking about Super Mario 64 DS?
You need 80 stars.