Luigi's Mansion Portrait Ghosts (Area 1)
In Luigi's Mansion the bosses are Portrait Ghosts or Gallery Ghosts, whichever of the two phrases you are more familiar with works. They're kinda interchangeable here and I would like to be in google for both terms ;). Anyway, this guide is a walkthrough for each of them. Just scroll down to whatever one you need; they're all in the order you'll encounter them in game and each has a picture with a very brief walkthrough on how to take them down.
For the very first Portrait Ghost - Neville, Wait until he yawns then you can hit him with the flash light. After you do that you'll be able to suck him up using the vacuum. As easy as they get!
For Portrait Ghost - Lydia what you'll want to do is use the vacuum on the curtain and then shine the light on Lydia as she turns around and is about to stand up.
For this Portrait Ghost - Chauncey what you'll want to do first is suck the horse in the room with the vacuum. This will cause Chauncey to teleport to the horse and the fight to start. In order to damage Chauncey and advance to the second stage you'll want to pick up the ball in the room with the vacuum and shoot it at him.
The final phase consists of three different parts. First Chauncey will throw giant rocking horses at you and you'll have to dodge them. Second thing he will do is summon a bunch of bouncing balls that you'll want to run around and dodge, one of them will stay behind however and you'll want to scoop this up and shoot it at Chauncey like you did to start the fight. If they hit you, one won't stay behind.
You'll want to use the ball that was left behind to throw at Chauncey himself, do this and he'll give you an opening to hit him with the vacuum. After you successfully damage him, he'll throw a tantrum and stomp around the battlefield and restart from the first part of phase 2 with the rocking horses.
Area 2 - Portrait Ghosts
For The Floating Whirlindas, the dancing couple, what you'll want to do is stand on the same tile they're on and face away from them in order to make them visible. They'll dance around for a little bit and then the male will speak and they'll bow exposing their hearts as seen in the screenshot above. This is when you'll use the vacuum obviously.
For the Portrait Ghost - Shivers what you need to do is light the candle he is carrying on fire and then chase him down into the Butler's Quarters. Once there wait for him to sit down and you'll be able to use the vacuum on him. Once Shivers is defeated the lights in the room will come on.
Portrait Ghost - Melody Pianissima; For this boss what you need to do is hit all of the instruments in the room so everything is playing the Mario theme then go over to the piano and speak to Melody. She will play you a song and ask you what game it is from, Super Mario Bros 3 is the correct answer, or at least was for me I have no clue if she changes questions.
Once the fight starts she throws pieces of paper at you, suck them up then suck her up with the vacuum. Nothing really special about her except the quiz part.
Portrait Ghost - Mr. Luggs; for this boss you'll want to light the candles on the table which will allow you to vacuum up his food. This will of course piss the fatty off and two waiters will come to replenish his food supply. Suck up the waiters and then finish sucking up his food and he'll start attacking you with fireballs. Dodge a select amount of them and he'll tire out, then you can suck him up with the vacuum.
Portrait Ghost - Spooky; for this boss what you have to do is get near Spooky to have him chase you around the yard. After he barks a few times Mr. Bones will come out and yell at his dog and throw a bone ghost into the yard. Defeat this ghost while avoiding the dog to get a bone to fall, as seen in my picture.
As Spooky licks this bone he'll be vulnerable to the vacuum. This is your chance! For mother Russia! After you defeat Spooky there will be a flower in the current area that you can water. It'll grow a bit bigger and start one of the longer side quests of Luigi's Mansion but definitely well worth it as it's extremely easy and rewards you with one of two Gold Diamonds!
Portrait Ghost - Bogmire; this is the final boss of area two so be ready! For this guy what I did was sucked up his black ghost friends and shot them at him when he was in his "spectral" form aka when you can't normally hit him. This sounds so racist I know. But when you shoot one of the black ghosts at him he'll go all bubbly and become vulnerable to the vacuum. This is your chance to suck him up!