Bill/William was working when the plan took place.
Mabel was dancing outside, yelling at the top of her lungs, "LALALALALA! LOOK! LOOK AT ME AND PAY ATTENTION TO WHAT I'M DOING! LOOK! LOOK AT ME! I'M A DEMON! I DO MAGIC AND STUFF! FIRE NOISES! LOOK! AT! ME!" and a whole bunch of other things.
She expected Bill to be offended and come out to yell at her, but he didn't do anything, just continue to swindle everybody. His boss and the customers were falling for his act. It was horrendous.
After about half an hour, Mabel stopped. For a moment, it seemed as if the little girl had given up. Not even a minute later, she returned and danced with a gallon of Mabel Juice nearby, with extra plastic dinosaurs in it.
It was five minutes before anyone reacted, but it was Stan and all he did was yell out the window.
It was time to bother Bill directly.
Mabel stared through any window closest to the demon in disguise and shouted, "Bill! Bill! Hey, Bill! Bill Cipher! Billy! Bill! Bill! Hey, Billy Cipher!"
"WHAT DO YOU WANT, SHOOT- MABEL?!"
"Your hat is stupid," she snickered. "Bill! Hey, Cipher! Billy!"
He yelled back, "WHAT IS IT THIS TIME?!"
"Hi. Hey! Billy Cipher! Billiam! Cill Bipher!"
"May I please be excused?" William finally asked Stan after enduring this for an hour.
Stan asked, "Why?"
"I'm going to shut her up."
"Good idea. She's scaring away the customers. Guess she wasn't fit for the job."
Bill Cipher was long gone, chasing the twelve year-old into the woods.
"GET BACK HERE SO I CAN STRANGLE YOU AND LEAVE YOU HANGING BY THE SWEATER ON A FLAMING TREE!"
Mabel was laughing. In sheer terror. This was actually a lot scarier than anticipated. And where was Dipper? He should be nearby, ready to exorcise the dream demon.
He was not.
Mabel ran straight into him, both of them collapsing to the ground.
"Dipper! Why aren't you ready?!"
"It took you two hours to get- RUN!" Dipper screamed a little too late.
Bill grabbed both of the twins by the back of their shirt. "YOU KIDS KEEP MESSING WITH ME!" he shouted. He actually spoke the way he normally did, his voice echoing for no reason. "THIS IS THE LAST STRAW. I CAN'T HAVE YOU-"
Dipper flung a handful of dirt into the demonic human's face. He let them go to wipe it off, giving Dipper exactly what he needed.
"Chaos magnum fecerunt, ut hominem! Demon triangulis ab sententia, egredere de mundo mortale!" he shouted.
The twins backed into the tree behind them, expecting the worst.
Bill also looked worried for a moment, standing still tense and with wide eyes. But a moment passed, then two, and several more, but nothing happened.
Bill started laughing, nervously at first but eventually losing it, "WOW, KID, YOU REALLY THROUGH JUST SAYING SOMETHING IN LATIN WOULD WORK? YOU'RE NOT RIGHT IN THE HEAD!"
"B-But the Journal-" Dipper stuttered.
"KID, I'M IMPRESSED YOU EVEN GOT HERE WITH THAT BOOK!" Bill kept laughing. "I MEAN, THE AUTHOR HAD NO IDEA WHAT-"
"Author?"
Bill began to say something, but his right sleeve was on fire.
"AAARGH! GET IT OFF! GET IT OFF!" he screamed, trying to put out the blue flames desperately, but it just spread. Eventually the entire physical form of the dream demon was on fire. "KIDS, YOU JUST DON'T UNDERSTAND THAT I WAS GOING TO DO YOU A FAVOR! NOW THIS. IS. HAPPENING!" The was a flash of fire.
Then he was gone.
"Let's never speak of this again," Dipper quickly said, staring at the smoldering Earth.
"Agreed!"