“Show it to me.” The adjudicator held up the coin, still red hot from her dragon-flame. She looked at each of them in turn, displaying the glowing sigil on both sides.
Twilight stepped forward, bashfully lowering the hem of her skirt, presenting her hip on which the sigil was emblazoned. Satisfied, she declared the contest begun. Before both wizards a magic seal faintly depicting their brands appeared.
The magicians drew their grimoires out before them with a wave of the hand, pages flipping rapidly as though they had been caught in the wind. Twilight stopped on one containing instructions to cast a simple defensive spell, modified by the Academy from an ancient invocation. The incantation had to be spoken aloud and, like most spells, it required a sacrifice to be cast.
“Reactive Field!” shouted Twilight, looking up from her book. Somewhere within her satchel a tiny vial of precious mineral powder vanished, and shortly after a shimmering mist appeared before her crest, obscuring it somewhat.
“You call that magic?” teased Trixie. Despite it being a relatively common spell, in truth Trixie had never heard of it.
It took Trixie a moment to find a spell to her liking, having liberated the overpriced tome from a flea market only shortly before the contest began.
“Sword of Shahryar!” she shouted, hoping the pronunciation wasn’t too far off. In her free hand the outline of a curved blade appeared, completely solid yet weighing less than a pegasus feather.
This being perhaps the first challenge of the competition, a small crowd of cautious competitors had begun to gather, hoping to learn a thing or two before they issue their own. Let them watch, thought Trixie, giggling to herself, for she had never successfully cast a spell like this before, and she was nothing if not proud.
She swung the blade, projecting a powerful arc of force that stirred the sand as it sped toward her rival. Then the shimmering fog Twilight had conjured drew itself together, intercepting the blow, the two spells canceling each other out and erupting with dazzling effect. When the storm of magic settled, Twilight’s crest was unharmed.
“You have got to be kidding me!” shouted Trixie angrily. She tried to swing the sword again, but it faded away along with most of her clothing, leaving her naked (aside from her hat and cape,) and dumbfounded.
The murmurs of the astonished onlookers shook her from her daze. She angrily clutched her cape to her chest and glared at them. “What are you looking at?” she screamed, scaring a few of them into silence. Her attention then turned to her rival.
From across the spell-torn sand she could see little miss dork-magician trying to suppress her amusement. “Is this your fault?” she shouted at her, trying unsuccessfully to conceal her breasts and her behind simultaneously.
Hearing that, Twilight burst out laughing, unable to restrain herself any longer. “I’m sorry.” she managed after a moment. “I don’t mean to be rude, but...” Twilight paused, tilting her head to the side. “Did you forget? I mean, that was no parlor trick you pulled off. Magic of that caliber demands a heavy price.” She shrugged and shook her head. “That must have been a pretty powerful spell to have burned through your materials and your clothes.”
“...Materials?” asked Trixie, her confused expression threatening to set Twilight to giggling all over again, causing her to cover her mouth and look away, something Trixie wished the onlookers would try.
Trixie began looking through her hastily borrowed grimoire, and saw written near the bottom of nearly every page what appeared to be a hand scrawled recipe, each demanding a number of specific ingredients and their rates of ‘equivalency,’ whatever that meant.
Even the most basic attack spell had a cost, and she couldn’t pay any of them. On top of that, she could only guess at the relative value of what she had, which at this point was very little.
It was Twilight’s turn to cast. Selecting an offensive spell this time, she subtracted another vial from her dwindling stock and closed her eyes, holding out her hand. The wind had begun to pick up, tossing her hair and skirt as it gathered about her.
“Gale Force Bolt!” she said, opening her eyes. The wind spiraled around her arm, then fired off toward Trixie with a terrible howl, scaring away a few of the bystanders. It hit with impressive force, dispersing in all directions and leaving behind worrying cracks in Trixie’s seal.
Trixie could not afford to retaliate, but she had to do something. She scanned her grimoire for anything she could cast, willing to sacrifice her remaining cloth so long as she could buy herself another turn.
At last, fortune seemed to smile on her when she stumbled across an unusual spell. She uttered the words ‘Bag of Wonders!’ with as much confidence as she could muster and retrieved her hat. She reached inside, producing a number of vials of mineral powder, seemingly from nowhere.
She held them up as the last of her clothing disappeared, hat included, leaving her utterly exposed, her lips pressed together in a forced, nervous smile. Trixie’s expression made Twilight feel like laughing again. Instead, she double checked her bag, finding that her bottles were missing.
Evidently, Trixie believed that she was now unable to cast, and perhaps even hoped to retaliate using the looted powder, but Twilight was nothing if not prepared. Closing her eyes, she quietly uttered: ‘Failsafe.’
The ingredients combusted in Trixie’s hands, causing her to drop them in alarm. Then the flames were drawn toward Twilight, shattering Trixie’s barrier and colliding with Twilight’s own, damaging but not breaking it. In one fell swoop, Twilight had won, managing to cast a spell using ingredients in her opponent’s possession. Even the adjudicator was impressed.
The crowd split up, some departing in search of their own exciting challenge, the rest remaining to gawk at Trxie, who had never been so humiliated.
Author's Note
I struggled for a while, unable to decide how I wanted the conflict to go, with each draft running longer than 1,000 words. As a result, many variations were explored and discarded, some barely resembling the finished work.
I ended up creating quite a bit of ’fluff’ detailing the background of the competition, as well as the ‘ins and outs’ of spell-casting in this alternate setting, most of which I was unable to include, (though some of it is present ‘behind-the-scenes.’) There were also a number of spells that didn’t make the cut.
Additional delays arose from trying to create a system of consistent rules for both the contest and magic in general that would still allow for the silliness I wanted to include in the battle(s) without (entirely) compromising the subtlety of the tale’s erotic themes, which have admittedly been quite diminished in favor of the story itself, which hinges on the progression of the duel.
Perhaps in the future I will find a place for those ‘world-building’ ideas, as I am not opposed to exploring this specific version of Equestria further which was, in fact, my original intention, but after so much time (during much of which this project was somewhat neglected in favor of others,) I felt I needed to finish this story, lest I give in to the urge to completely rewrite it, (again.)
I am not entirely unsatisfied with it, taken as-is, and I hope you will find some enjoyment as well; and if you actually took the time to read my long notes on this short story, I applaud you, and apologize for the mess. 
