The instant Haku put his hands together, making a strange hand sign that Sakura didn't know, the ambient temperature began dropping quickly. Each time she exhaled, she would see a small cloud of condensed water vapour forming in front of her face…
"Secret Jutsu: Demonic Mirroring Ice Crystals!"
The water puddles under the two boys' feet immediately froze solid, as the temperature dropped below zero. Pillars of ice first began forming all around them, and as Haku's Jutsu continued gathering momentum, the very water in the air began condensing into thick, interconnected panes of ice; Sasuke and Haku were trapped together inside the igloo!
Kakashi's eyes widened; he didn't know this Jutsu! And more likely than not, it was the work of a Kekkei Genkai; not even his Sharingan would allow him to unravel such a technique! He knew Sasuke wasn't ready to face someone with such a mastery of the elements, so he immediately tried to run to his rescue…
"Sasuke!"
"Let the brats have their fights, Kakashi! Your opponent is right here!"
Zabuza flickered into Kakashi's path and swung his Kubikiribōchō, forcing him to back off; Sasuke would have to fend off Haku on his own… but unfortunately for Haku, Sakura had been waiting for him to exhaust himself by using a large-scale secret technique like his ice mirrors!
"You stay here, Tazuna-san," Sakura told the old bridge builder. "I've got to help Sasuke out of a bind."
Tazuna nodded, so Sakura darted towards the floating ice panes, weaving hand signs as she ran.
"Sasuke!" she cried. "Mirror my moves!"
Sasuke nodded, and he activated his One-Tomoe Sharingan; he was quicker at making hand seals than she was, so he quickly caught up to her. Sasuke and Sakura both took in a deep breath, and…
"Fire Style: Great Fireball Jutsu!"
"Fire Style: Great Fireball Jutsu!"
It was hard to tell what Haku was thinking, with his face hidden behind his mask, but somehow, Sakura could tell that he wasn't worried in the slightest.
Sakura and Sasuke continued blowing flames at the ice. At first, nothing seemed to be happening, but then, a cracked formed; and then a second, a third, and then hundreds at once… Haku, who was hiding in one of the mirrors, desperately infused more chakra into his Jutsu, but his ice was quickly weakening and melting, until finally, the mirrors all came crashing down, shattering into pieces!
"How!?" Haku panted, as he was forced out of his ice mirrors. "Fire has never worked against this Jutsu…!?"
It was simple thermodynamics, really: at normal atmospheric pressures (101.3 kPa), ice will not directly sublimate into gas at high temperatures; it must first turn into a liquid, even if it is only for a fraction of a second. But before turning into a liquid, ice must undergo the melting process: its temperature will remain locked at zero degrees Celsius, until its particles accumulate enough energy to change its phase.
Assuming that Haku and Sasuke were fighting alone, and that equal amounts of chakra were being input into both techniques, ice would inevitably win over fire because of one simple reason:
The heat from Sasuke's Great Fireball Jutsu would first begin melting the outside of Haku's Ice Mirrors, but since ice must first turn to liquid before evaporating, the mirrors would technically not lose any mass. Then, since both hot and cold are at an equilibrium, the cold from his Haku's ice nature chakra would resolidify the melted water back into ice… except this time, the ice would become even smoother, with even fewer imperfections!
Tempered by the flames, the newly reformed ice would have even less surface area for Sasuke's flames to attack, making Haku's mirror increasingly more resistant to flames; a perfect shield against Fire Style Ninjutsu! And since Sasuke would run out of breath before he ran out of chakra, he would never be able to break through this shield…
(I'm so sorry I made you read about science in my ridiculous story about expanding breasts in Naruto's world)
Unfortunately for Haku, his Demonic Mirroring Ice Crystal Mirrors were being attacked from both sides; Sasuke from the inside, and Sakura from the outside. The equilibrium was broken, as Haku's chakra was not powerful enough to cool down both sides of his mirrors in response to their fireballs.
And thus, Haku's Jutsu failed, allowing Sasuke to escape!
Haku flipped backwards and landed lightly on his feet, apparently no worse for wear despite having his Jutsu broken. But from his ragged breathing, Sakura could tell that he had wasted much more chakra by creating and then maintaining his ice mirrors than he'd erstwhile intended…
Meanwhile, the continuous clanging sounds of metal weaponry banging against each other could be heard farther out in the mist; Kakashi and Zabuza seemed about on par with each other, at least while they were in the mist. Every so often, Sakura would catch a glimpse of their figures through the mist out of the corner of her eye, but they would vanish just as quickly as they had appeared…
Suddenly, the sounds seemed to cease, replaced with an eerie calm. Haku, Sasuke and Sakura froze in place; the sound of footsteps was rapidly approaching them, but would it be friend or foe?
"YO!" came Naruto's obnoxious voice. "The hero always arrives fashionably late, ya know?"
The footsteps grew louder and louder, until Naruto finally burst through the mist with a flying kick aimed straight at Haku's face.
'D-d-d-dynamic entry!?' Inner Sakura sputtered.
Haku raised his arms to block, but the impact from Naruto's kick penetrated through his defences, shattering his mask. He took a few steps back, panting heavily, before flinging at Naruto the eight senbon he'd been holding onto between his fingers, forcing him back as well.
"Haku…" came Zabuza's chilling voice through the fog. "You had every opportunity to kill the boy, but now your decision to spare him has come back to haunt you; you might not live to learn from your mistakes, at this rate…"
Things were looking bad for Haku; his chakra was nearly exhausted, and it was three versus one.
"I know that," Haku impassively called back. "But even so, Zabuza-sama, I want to prove myself a useful tool to you. Please let me take care of them on my own…"
The air started turning cold again.
"It's you!" Naruto gasped. "You're that flowers girl! I mean, you're the guy I met in the forest!"