Cow her into submitting to the deal

You shrug. "Suit yourself. But before we begin.. Is this yours?"

You hold out your arm to show her the large-cupped bra, pink with frills, which you removed from her body the moment she said 'fat chance'. The woman's eyes go wide with recognition, an expression you've seen hundreds of times but is still enjoyable every time, as a hint of blush starts to creep into her cheeks. She glances down, finding her leather top hanging unbuttoned and open, revealing an intolerably large part of the curvature of her ample breasts, her wide pink areolas still narrowly concealed beneath the fabric but liable to be exposed at the merest shifting.

Her arms shoot towards her top, pulling it tightly closed in a huff.

"This.. This is all just a big bluff. Spells like these, this has gotta be the last of your mana!"

"If it is, then why are your pants undone?" You reply with a wink and a smirk. The woman looks down, and her hands try finishing to rebutton her top in a growing panick before moving to the open crotch of her tightfitting pants, exposing the frills of her pink panties. Before her hands manage to reach the waistline of her pants though you've already made your next move, appearing behind her to peel her pants halfway down her legs.

The plus-sized woman lets out an aggrevated grunt, spinning around trying to land a blow on you with her fist, but she doesn't stand a chance. As soon as she's turned around, legs hobbled by her pants, you're long gone. A split second after she realizes that though, she also realizes her top is unbuttoned - again. You've now stepped up behind her to slip a finger over the waistband of her panties, which you can now see have been worn into the shape of a thong from the size of her curvateous ass. You pull your finger back, and let the thing snap loudly against her skin, eliciting an "eep!" The woman spins about, visibly frustrated, simultaneously trying to keep her top shut and pulling her pants back up. You can hear the three boys watching this excitedly watching and laughing at her predicament.

"Forget about limited mana, because I can do this all day, sweetie, and I intend to do it to you for at least another 24 hours."

You've reappeared next to the three dangling girls. A little while ago you had painted S P A N K ! across their six collective butt cheeks, and you stand next to the exclamationpoint, pretending to look pensive.

"I know that technically I already ended the sentence with this punctuationmark here.." you tap the aristocratic girl's ass with the exclamation point "...but if I hung you fourth in line and painted an M on one of your cheeks and an E and another ! on the other I don't think observers will have a problem with that. Then it will spell SPANK! ME!, but with extra emphasis. What do you think?"

You manipulate time to appear right behind her again, and gently tap her butt three times. "M, E, ! Right there." Then you vanish again to reappear a few meters in front of her.

"So what will it be, honey? Heroism or shared humiliation?"

The woman, visibly fretting, exchanges glances with the three hanging girls, who are trying to look back at her over their shoulders hoping she'll get them set free. There's an audible gulp and you hear the humiliation and resignation in her voice even as she tries to keep up a strong front. "24 hours"

At that moment the three girls find their wrists freed, and their feet hit the floor. They quickly cover themselves and scatter in various directions, the word painted on their asses breaking up with them.

"Good choice"