Now guys, before we go on, I think I should probably apologize for the somewhat indecent themes going on here. I mean, I feel like Nintendo doesn't usually put this much poop in their games.
...But I'm NOT going to apologize, since I never promised I was going to be mature about this.
COMMENCE THE BOSS FIGHT!!

Luigi: Is this that 'playtime' you promised me a while back?

Author: Of course. Completely intentional.

Luigi*crosses arms*: Well I don't want to play with bouncy balls! ...I just want Kevin back!

Author: Too bad Luigi! You stay and play with the toys you're given like a good boy!

Mansion: Yeah, B O Y. This is playtime.

Author: ...

Mansion: Start your engines.

Author: Uh, Wrong kind of —

Mansion: YOU DARE CORRECT THE OH GREAT AND POWERFUL MANSION!?

Author: *cowers in the writing corner*

Anyway, Chauncey grabbed a random bucket and promptly started pitching diapers into the crib like it was the great pie war of '72. Whatever that is.

Chauncey: The great Poop War of —!

Author: Okay Nevermind!

"Aaaahahahaahhaa!! How am I going to explain these stains to the final boss?!" Luigi hollered, running around the bed like a dust mite. "And who keeps their own poop in a bucket, anyway??"
...
Since he was actually only two inches tall, the diaper bombs weighed more than he did, and he was sent flailing up in the air every few seconds as each one landed.
Man. Can you, like, imagine how terrible this would be if someone took this out of context.
"Shush up, Twilight Sparkle! I already told you if you make this hard I'll have to call backup!!"
Luigi stopped mid-run to frown. "No you didn't!"
He was promptly beheaded by a giant bouncy ball. (Not really. But it did wipe him out.)
With a wave of his very manly rattle, Chauncey sent another army of balls into the crib, which sloshed around like inflatable roast turkeys. Then another rain of rocking horses crashed into the mattress.
"KEVIN!! THEY'RE ALL KEVIN! THE WORLD HAS GONE MAD! EVERYWHERE I LOOK I SEE DEAD KEVINS!!" Luigi shrieked.

~Elsewhere~

Boolossus jerked up from the royal Keurig. "Did somebody just mention me??"
Gumboo looked up from polishing the royal coffee mugs(which looked like big, fat boos with handles), made a weird face and said, "Is your brain bilocating again?"
Boolossus frowned. "Oh. No, Nevermind. I was just listening in on the narrator again.
...And my name isn't even Kevin."
Then King Boo's voice thundered from the next room. "KEVIN!! WHERES MY ESPRESSO?!"
This sudden outburst caused Boolossus to jerk forward, get his stub caught in some unholy crevice of the coffee machine, and scream wildly in response while Gumboo dropped the glasses, shattered them all over the floor, and pathetically tried to assist getting Boolossus out. It all was over in a matter of twelve seconds and many cringe-inducing images.
And then the coffee all poured out onto the floor.
So it turns out Luigi's not the only one who has a screwy life.

Luigi: It makes me feel fuzzy inside. :)

After that random intrusion, you might redirect your attention back to the crib colosseum with Luigi the Cheerio. For a while that diaper-throwing competition went on, a classic pastime for those of you who had siblings, and then Luigi grabbed one of the bouncy balls, pelted Chauncey with it like a 5th grade bully, and lit him on fire because the kid wasn't watching his cigarette. Then he sucked Chauncey away, won himself a key that was as girly as everything else in the nursery, and that was the end of the first boss.
And so boys and girls, this is a lesson not to be careless with your cigarette butts.

Toad: Wait. That was it?? What was that?!

Author: That was the boss.

Toad: Seriously? I thought it would be longer and more... pathetic.

Luigi: Yeah, I thought so too, but we have like an attention span to uphold and at the rate we're going, this book isn't gonna be any less than 50 chapters.

Author: No! It's gonna be no more than 40! So there, watch me!

Luigi: *starts banging head on wall, the sheet rock gives and his head goes right through*

Mansion: My sentiments exactly. Minus the putting a hole in me part.

Anyway, with the dead baby freshly put away, Luigi was magically restored to full size in a magic swirl of smoke.
He just wasn't removed from the crib, so the next thing they knew, Luigi was stuck in the bars in a tangle of legs and Mappy argued and coached him while he had to use the Poltergust's nozzle as a crowbar to free himself. I believe that is what you call domestic procedures.
Anyway, with the drama finally put away, E. Gadd decided to intrude on our barely-restored peace of mind.
MEEMAWMEEMAWMEEMAW!!
"Aw, just when the stench was clearing," Luigi complained, answering the Game Boy Horror.
"Uh, Luigi, that stench is still you," Mappy whispered.
"What?? No it's not!" He proceeded to sniff his underarm and then fainted.

Author: Note to self — give Luigi a bath later.
That's called foreshadowing, folks! :D

Then Luigi picked himself up. "What do you want, Gadd?!" he barked for some reason.
"...Luigi, can you hear me?" the professor asked, pushing random buttons.
"Unfortunately," Luigi replied.
"What was that? I lost your connection for a sec... a few minutes actually. I was a tad worried."
Luigi stopped. "Really?"
"No, I said a tad. That means almost not at all."
The Mansion suddenly began to shake momentarily with a deep chuckle of laughter. "Oh, Luigi... if only there was someone out there who loved you!"
(Ooooof. Burn.)
Luigi stopped and gasped in disbelief at the random voice of the house.
"...There is someone who loves me! Very very much loves me!"
Mappy groaned. "If it's your mom I'm gonna cry."
Luigi reddened. "No! ...It's your mom!!"
(Audience makes low sounds and murmurs.)
Luigi frowned. "There's... other people too! Out there! Maybe even in here!"
Mansion rumbled like a... giant building that was rumbling. "If you're referring to your brother you can go ahead and forget that he exists as a person."
Luigi gasped and turned in circles. "Gadd, the mansion's threatening me!!" he hollered.
E. Gadd frowned. "Excuse me, Gadd? ...Did you just call me Gadd? Don't ever. call me. Gadd."
Luigi just stared, wondering if he should suddenly be afraid.
Then E. Gadd jumped up. "So Anyway! Looshy! Why don't you come on back to the lab? You must be pretty tuckered out after that battle, huh?"
"Uhh, actually I'm feeling fine now, thanks! How'd you even know I was battling something, anyway??"
"I could hear the screams and smell the diapers from all the way out here," E. Gadd replied.

Luigi: Man, it's a good thing there's no one else in the neighborhood.

King Boo: ...

"Yeah, I just caught myself a BOSS ghost," Luigi boasted. "And I even found myself this little... pink jewelry box key!" he flaunted it in the camera. "I bet it opens, like, an ancient sepulcher."
E. Gadd waved it off. "Yeah that's great. Why don't you come on back to the lab for a second so I can empty the Poltergust? When you vacuum a boss ghost, it gets pretty much full to bursting. You wouldn't want that, would you? Huh? Huh? Huh?"
Luigi frowned. "What do you mean? I thought a vacuum was like... a black hole."
E. Gadd stared. And he put his head in his hands. And he didn't come out for a real long time.

17 seconds later, Luigi walked out into the foyer and headed for the front doors. He was halfway out when Toad looked up from his Pokémon.
"Hey Luigi, whatcha doin??"
"Uh... leaving the mansion because an annoying bozo professor told me to."
"Oh. Well, don't stay out too late. Be back here by 3 am, you hear?? Mario's not gonna save himself."
Luigi frowned. "Yeah, actually, do we know that? Maybe he's just gonna —
"3 AM, LUIGI!!" Toad hollered. So Luigi hightailed it out of there.

Meanwhile, somewhere across the dimensional barriers which are called brick walls...

An Unknown Villain hovered over a glowing orb which was projecting an image as he gazed into it. There before him was the very scene we ourselves saw, Luigi heading obliviously down the steps of the Mansion. Seeing this, he chuckled.
"Well, would you look at that, Mario," he snickered over his shoulder. "Your little savior is running away. Leaving the Mansion and skipping like a six year old as he does!"
Mario frowned from an unspecified place. "You mean Luigi? He always skips like that."
Secret Captor frowned and glared at him. "Oh, someone do shut up his mouthpart. It's annoying."
Taboo, who was among the row of boos randomly lining the wall in uselessness, floated forward and yanked out a roll of masking tape.
Mario jerked up. "Whoa, hold on a second, let's talk about this! Or... not talk about this! Don't you dare put that near my face, I have a mustache and it's not gonna —

Well, I think it's a good time for a scene change.

When Luigi walked into E. Gadd's lab he was greeted with terrifying music which was a ripoff of Elmo's World, only the words were 'E. Gadd's Lab'. It's a real bop.
Here, mentally insert the words yourself.

[There should be a GIF or video here. Update the app now to see it.]

My condolences.
Luigi stood there in annoyance. "Why exactly did you drag me back here??"

Mappy: Oh, So you WANT to be in the Mansion now??

Luigi: *sigh* Well I'm starting to think any place is better than here.

E. Gadd finished jumping up and down like a totally decrepit old person and ran over. "Oh, Louie! You made it back in one piece! I wanted to see what you thought of my new lab theme song, to replace the old background music! What do you think?"
Luigi glared down at him like a moody teenager at 4 am. It's not becoming of his face, but he kept it up for 32 seconds.
Then E. Gadd flipped the music off. "Uh, anyway. Come on downstairs, I have to show you something."
Luigi was exasperated. "What is it now, a time machine so I can go back to before I knew you?"
"No, that's coming after the alien invasion."
"WHAT?!"
"Anyway, take a gander at this!" E. Gadd declared, slapping a dumpy machine that looked like a giant makeshift mousetrap fashioned by untrained men with soldering guns. When he hit it, a random coil of hose popped off and ricocheted somewhere. "...That probably wasn't important."
Luigi raised an eyebrow. "...What is it? An oversized bubble machine?"
"No... this fine piece of work took nearly twenty years to develop. Twenty valuable years. Which were probably much better spent then your last twenty," he muttered.
"Hey! Those were extremely formative years!" Luigi yelled.
"Whatever. This lovely machine is my most valuable possession, the crowning jewel of my achievements which I will definitely never, ever in a million years get rid of."
"Yeah, right. That's your greatest achievement?... I figured," Luigi muttered.
E. Gadd ignored him. "I call her the Ghost Portrificationizer!"
"The Ghost Potty with hand sanitizer? Oh good. I was worried we didn't have one of those. Speaking of, do ghosts use the bathroom anyway?"
E. Gadd ignored him. "How does it work, you ask? Well, you just plug the Poltergust into that slot over there and sit back and enjoy the ride!"
Luigi frowned. "I didn't ask that," he replied, walking over to observe the slot. "What is this made of, a recycled 50's washing machine?"
"That's classified. That thing turns the ghosts you catch into paintings."
"Oh, so you mean like, body art."
"Just stick the thing in!!!"
"Fine!" Luigi hesitantly raised the nozzle to the slot, then almost died when it's suction suddenly pulled him closer.
"AHHH! PROFESSOR HELP ITS PULLING ME IN!!"
"That's just the fan, Loogie, sheesh. Now follow me! The ghosts are invisible..." he began, indicating a giant window filled with liquid. Inside they could clearly see three blobish spirits.
"No they're not, I can see them right there," Luigi pointed.
E. Gadd ignored him, walking down the line to a conveyor belt. "So we need to press em into visible forms..."
Luigi stared as three picture frames were rolled out, flattened ghost shapes were crushed, zapped by overhanging lasers, and then fitted into the frames like cross stitches. Then they were rolled into a giant broiling oven to be baked.
Luigi blinked in behorrorment. "You do this to all the dead ghosts??"
"...And presto! Turn em into beautiful ghost portraits!!" E. Gadd proudly held up a picture of Lydia, who was surrounded by cosmetics and sneering at the viewer with a curling iron in one hand and an upraised cowboy boot in the other.
Luigi turned white like raw chicken. "Is-s that what y-you do to us when we die??
...That's not humane...
THATS TORTURE!!"
Bolting upright, he ran screaming like a banshee towards the exit. "ITS AN UNDERGROUND TORTURE CHAMBER AND WE'RE ALL HEADED TO BE EXPLOITED FOR THIS MAD SCIENTIST'S DEMENTED EXPERIMENTS LIKE IN THE SECOND WORLD WAR!! AAAHHHH!!!"
E. Gadd just stood there in relative chill as he let Luigi panic like a five year old. When the paintings were done, he collected them all and shrugged.
"I already explained it to you in my overly long and detailed horrifying story," he said. "Don't you get it? It's just how these things have to be done. They have to be."

Author: Oh yeah, the restless deceased HAVE to be stored and displayed in frames on people's walls to prevent the zombie apocalypse that we're always struggling to hold at bay, don't you know?

Luigi quivered in the corner. "B-But! THE LASERS!"
"They can't feel it, you know... they're dead! And since they're dead, that makes it right to do whatever I want with them," the professor reasoned aloud.
It's nice to know that we have good protagonists in this story who uphold good morals and are not antagonistic in any way.

Neville: Hey! We can hear you, you know!!

E. Gadd: No you can't!

Lydia: I swear when I get out of here again I'm gonna make sure you never recognize your own face in the mirror! And also your hairstyle is atrocious! When you last went to the barber —

Chauncey: ...Morons.

Lydia: ...Pardon me?

Chauncey: ...Goo goo ga ga?

So anyway, sickened by all this at once, Luigi quickly headed back outside for the relative safety and security of the Mansion.
So, sucks for Neville and his family, but Luigi's more important.
"Alright Luigi, what's the plan now?" Mappy asked as they headed towards the giant steps. Mansion glared down at them with his glowing yellow eyes of sinister secrets, but he didn't say anything.
I mean I'm sure he thinks many things behind those stony windows, but eventually one has to come to terms with the fact that they're an immobile dwelling place of brick.
"I have this little girl's jewelry box key thing," Luigi said. "This is definitely gonna be really important!"
Mappy groaned. "Luigi, I know you got that from the boss, but that thing is in the shape of a pink heart, I seriously doubt —
All was lost when they entered into the foyer to the giant pair of doors with a big heart across them.
You know, the one that's right in front of your face when you first step inside, screaming 'OPEN ME NOW'.
"Were you saying something?" Luigi asked.
"I think we should hurry up," Mappy replied.
Toad was sitting there so engrossed in his game now that he didn't bother looking up. As Luigi approached the door, the glowing thorny vines that looked definitely poisonous started flashing violently (this was to kill off any weak-hearted interlopers who would otherwise waste Mansion's time). However, by some very curious trick of the stars, Luigi was unaffected.
The vines fell dead to the floor.
Luigi gasped. "Did you see that?? It opened! I must be an incredible house whisperer or something!"
Mappy rolled his eyes. "Luigi, the Mansion is leading you," he said. "So you can get killed."
Luigi ignored him, already lost in a dreamy musical of magic and romance.
"Love is an open DOOOOOOOOOORR!!!" he screamed maniacally, busting through the heart doors and into a dim dark hallway. Without even stopping to acquaint himself with the maze of corridors, he pranced forward, making some random turns now and then. "ALL MY LIFE HAS BEEN A SERIES OF DOOOOOORS IN MY FACE!! AND NOW SUDDENLY THEYVE BEEN MULTIPLIED!!
...CAUSE I'VE BEEN SEARCHING MY WHOLE LIFE TO FIND MY OWN PLACE!!"
The Mansion suddenly jumped in with a deep tenor. "AND MAYBE IT'S JUST A WEIRD LETTER THAT BROUGHT YOU TO MY SIDE!"
Luigi looked up and grinned. "But in youuuuuuuuuu I've found my place!"
"—This haunted space!"
"And it's nothing like IVE EVER KNOWN BEFOOOOOOORE!!!
Finally an open DOOOOOOOOOOOORRR—!"
"Luigi!" Mappy cut in harshly, ruining the tender musical and sending the music screeching to a halt. "Can you fraternize with the enemy later?? And spare all the lucky people who haven't seen Frozen!!"
Luigi slumped and kept walking very boringly down the hall. "I was just expressing my pleasant surprise at seeing an open door! And besides, I wasn't fraternizing! I don't even do that with my own brother!"
"Hence many of our current problems."
Luigi snorted. "What are you saying, I should just go... fraternizing with him??"
"Ideally, yes! When all this junk is over!"
Suddenly the Mansion laughed. "Oh, so sad... there won't be any room left for fraternizing when I'm through with you tonight!"
Luigi gasped. "But Oh Great and Powerful Mansion! I thought we were buds! We sang our song about our newfound connection!"
Mappy frowned. "...Luigi, have you even seen Frozen??"