In the time that Naruto and Sakura's battle with their opponent, the demon brothers were easily taken down by Lu, Sasuke, and Kakashi due to the overwhelming power Kakashi showcased over the demon brothers. As they were about to confirm whether the brothers were knocked unconscious or killed, a nearly suffocating veil of killing intent coupled with mist engulfed them. Kakashi was used to the level of killing intent he was feeling, but he was worried that Lu and Sasuke would be put out of commission due to the sudden exposure to a jonin-level opponent's killing intent. Furthermore, if they were put out of commission and there was even one more enemy, they'd struggle with protecting Tazuna or surviving the mission. Considering his options and that he didn't lose much chakra, he attempted to track any chakra in the mist, but it was obscured because the entire mist was created by chakra from the presumed enemy.
"I..t's…" Sasuke choked, "Za…buz…a," he struggled to tell Kakashi, but all it did for the man was confirm his thoughts. As for Sasuke overcoming the killing intent he was practically suffocating from, he forced memories of the night of the Uchiha massacre, which overwhelmed him with sorrow. Still, it reminded him that he's experienced a worse feeling than a jonin's killing intent, the feeling of betrayal after seeing everyone he held dear to him being killed in one night by the one person he idolized.
Lu coughed and grasped the dirt with all of his strength while attempting to recall how imposing Orochimaru felt to him nearly eight years ago, but he had no real memories of the man. It took him a few minutes to stabilize himself, so he recovered shortly after Sasuke did.
"Oh?" a voice called out from the mist, "I'm surprised you boys can stand. I intended to knock you both unconscious to save you the trouble of seeing what was going to happen to your sensei, or yourselves for that matter," the man chuckled.
"Sasuke," Lu whispered, looking to his left at the boy he was calling. Upon noticing he'd got Sasuke's attention, Lu signaled for Sasuke to follow his lead. Following that, Lu built up some chakra in his chest and expelled a huge bubble of chakra that absorbed the mist where he and Sasuke were, opening a path to where Naruto and Sakura were. The idea Lu had in mind was to get Naruto to use the wind style: wind palm jutsu to dispel the mist. Staying with Kakashi in a situation like that would do nothing but inconvenience him and get them all eventually killed.
"The brats left you, Kakashi," Zabuza chuckled from the mist, " I guess they had no guts after all," he taunted.
"Spare me the banter, Zabuza, Demon of The Mist," Kakashi lazily replied, before building chakra in his hands and chest, molding signs for a fire style: fireball jutsu, and expelling the jutsu into the mist, noting that it was sucked up immediately as if the mist had some sort of anti-chakra property.
"I'm sure you didn't just throw a shoddy fireball into the mist for no reason," Zabuza's voice echoed in the forest, before he built up chakra of his own so that he could cast the water release: water dragon bullet technique at Kakashi to distract him and throw a kunai at the back of his neck, which the copy ninja easily deflected.
As Sasuke and Lu were approaching the area they felt the chakra signatures of Sakura and Naruto, Sasuke was wondering if the plan would help as much as Lu claimed it would.
"Are you sure you can get us out of this situation by getting Naruto and Sak-" the Uchiha was cut off. An immense wave was barreling near Sasuke and Lu, along with the barrier they presumed to be Naruto's, dropping.
"WOOOOOOO!" Naruto excitedly called atop the wave.
Sasuke and Lu looked at each other and shunshinned into the highest point within the trees and saw that Naruto was standing atop a log and practically surfing toward Lu, Sasuke, Kakashi, and Zabuza.
"You're shitting me," Lu grimaced, before creating a huge bubble to absorb any of the water that came in contact with him or Sasuke, siphoning the water into his chakra, and allowing any water that wasn't being actively infused with chakra onto the forest clearing, creating a large puddle below himself and Sasuke. Sakura was sitting on the log that Naruto was standing on, using her chakra control to stay planted on the water and slightly control the water surrounding the log to keep it somewhat stable.
"Where does he come up with these things?" Sasuke, in annoyed astonishment, commented.
"I have no clue. Not one," Lu replied before they both trailed behind Sakura and Naruto, returning to Zabuza, Tazuna, and Kakashi noticing the once thick veil of mist was momentarily dispersed.
The genin took action as soon as they got reunited with each other. Sakura was supporting Lu, Sasuke was backing Kakashi, and Naruto was actively inconveniencing Zabuza with his clones and constant lightning balls. Sasuke was following up attacks and somewhat coordinating with Kakashi, but it seemed to be slowing his sensei down a bit so he stepped back and assisted Lu and Sakura. They were planning to catch the man off guard by using Lu's charka draining bubbles in a similar way that Sakura could her water bullet jutsu; the issue with doing so was that Lu couldn't condense them enough whilst also retaining the bubble.
"I don't think that'll work," Lu sighed, "I need to keep it as a bubble style technique, but condensing the jutsu and also trying to shoot it in a way you would a water bullet would likely not allow it," he explained.
"Instead of one bullet, why not use it like a stream of condensed bubbles the same way I use the wild water jutsu?" One of Naruto's clones suggested from a distance before casting another wave of lightning balls, hurling them behind Zabuza as he dodged Kakashi, tagging the man and slowing him down a little.
"You know that strangely makes sense, and considering Naruto probably has more than half of his chakra remaining, we're going to have to come up with a plan that involves all four of us and maybe Kakashi sensei, if that doesn't slow him down," Sakura added.
With the options open, they formulated a plan to restrict Zabuza and it would involve Kakashi letting down his guard and their opponent underestimating them. The first step would be to get Kakashi out of commission in some way that implied he was unconscious, the next step would be to have Sakura fire multiple water bullets at the man so they could throw him off, and as he retaliated, get Sasuke, using the fire style: flame fist, to engage in taijutsu long enough for Sakura to throw a volley of shuriken, with two demon wind shuriken, transformed formed of both Lu and Naruto, to then have Naruto blind the man using the water style: wild water jutsu, and Lu following up by condensing small bubbles into a cloud-like fume that would weaken Zabuza, distracting him from Kakashi, who should be at least in better condition, and engage Zabuza with Sasuke and Sakura, while Lu and Naruto make sure Zabuza wouldn't have a last-ditch effort and attempt to kill Tazuna in his last moments. Tazuna, for his part, was fine for the most part, aside from being terrified.
"Alright, we only have one shot at this, so you," Sakura said pointing at the clone that was helping them plan, "...should dispel at this point. Thanks for helping us, but if you could dispel and have Naruto send another clone to tell Kakashi the plan, that'd be great," she finished.
The clone saluted her and did as she asked, bringing a slight smile to her face. Kakashi's battle with Zabuza wasn't too difficult for the copy ninja, especially because of the number of obstacles his students were throwing Zabuza's way, so when he got the message, he was surprised they wanted to take a risk like that, however, he shrugged as it gave him more time to be lazy and he led the battle to the puddle Naruto created. Zabuza got near the water and immediately began molding hand signs for the water release: great waterfall technique, but Kakashi did the same. The mercenary realized he was falling for a genjutsu because of the copy ninja's sharingan and sucked his teeth in annoyance.
"I truly wish you'd die already," Kakashi and Zabuza complained in unison, but it was clear the actual comment itself was initially Zabuza's.
"Seriously? And now you're mocking me?" They both scoffed in unison.
"Water style: great waterfall jutsu," they chanted, releasing their hand signs simultaneously, throwing equally potent large spirals of water at each other, presumably knocking Kakashi out of commission.
"You weren't much, after all," Zabuza scoffed before going down to confirm his kill.
"Not so fast, Zabuza," Sakura interrupted, sending four water bullets at the man, made by her canceling the jutsu early to send out a barrage of water bullets rather than an assassination arrow meant to incapacitate her opponent, which he easily dodged at the expense of creating distance from Kakashi. Before Zabuza could respond, he was stabbed in his back, but unfortunately for the genin, it was a water clone. Sasuke leaped down near Kakashi and was attacked suddenly by Zabuza, forcing him to engage with the man in taijutsu, using the fire style: fire fist jutsu to compete with the man. Sakura then threw a volley of shuriken at Zabuza, with the included false demon wind shuriken, Lu, who, before transforming, released a few large bubbles to detect Zabuza's real location. He then instructed Sakura to throw himself and Naruto in that direction, and Sasuke followed Lu in Shuriken form with a converted Naruto in his own, throwing Lu off course after ricocheting Naruto off of him. Zabuza deflected Naruto's transformed shuriken behind himself and Lu, above Zabuza, undid his transformation and released a cloud of bubbles, stunning Zabuza and sapping his chakra. Naruto returned to normal and signaled for Sasuke to use the rat tremor jutsu when Naruto used his wild water wave jutsu, blasting Zabuza with the combination of both jutsu, staggering the man.
"I've gotta give you brats some props, you've inconvenienced me greatly," Zabuza complimented through staggered breaths. "But, I'm not some scrub who would die on a mission to a bunch of children, no older than the ones I killed to graduate and become a shinobi…which is exactly…" he trailed off, "That doesn't matter anymore," he grumbled. "Ninpo, Hidden Mist Jutsu," the man whispered. He followed up by creating six water clones, which all hurled water dragon bullets at the genin.
By this point, it was unavoidable for Kakashi to be involved with the battle and he began to build up chakra. The genin backed up to each other and came up with the plan to all use the head hunter jutsu, despite Zabuza still being able to sense them, it would help them avoid the barrage, so Lu and Naruto threw down smoke bombs and the four of them hid. Nearly fifteen seconds later, Lu resurfaced and formed a protective bubble around himself and his genin comrades, dubbing the jutsu the "bubble-style: expansion," which was followed by Sakura using a very thin terra shield that was reinforced by Naruto placing a seal with his chakra infused into the ink, holding it in place momentarily. After this Naruto did the hand seals snake, ram, boar, horse, and bird, clasping his hands together to form the gale palm jutsu, which was released in each hand to his sides as he turned quickly, forcing a small clearing in the mist where Lu and Sasuke could see seven Zabuzas, throwing three explosive kunai each in between each Zabuza, catching two clones off guard. Unfortunately, the man threw his blade, which was similar to that of a guillotine, right at Sasuke, pinning his throat between the blade and the cusp of it, essentially trapping the young Uchiha.
"I can't die here," Sasuke coughed, substituting himself with a nearby log, falling to his knees as he recovered. He gasped for air and realized his vision was blurry but also clearer than usual, he couldn't understand what he was seeing and overwhelmed with exhaustion, fell unconscious. The clone that did this to the Uchiha was cut clean in half by Kakashi, who grabbed the boy and shunshinned him a few meters away from the active battle.
Naruto watched closely as he made sure Sasuke was still breathing, but because he wasn't paying attention, Zabuza, or at least one of his clones took the liberty of landing a swirling back kick intended to hit his chest, but was blocked by his right arm, sending him flying and causing him to cough up blood and fall at the base of a nearby tree. The clone followed up by taking a kunai and lifting Naruto by his now tangled right arm, nearing his throat with the blade, but returned to water form, dropping the boy, after being pierced through the chest by a red chain that trailed off from the boy. Kakashi was engaged with another clone, this time, defending Lu, who fell back and was nearly unconscious. Kakashi saw from the corner of his eye that Lu was brimming with lightning nature chakra and was expelling an alarming amount of bubbles about the size of an eyeball around his body, like a cocoon. Once Kakashi finished off this clone, there were only two clones remaining and Sakura was still conscious, although frightened. Kakashi quickly shunshinned both Naruto and Lu to where he hid Sasuke and instructed Sakura to stay near them. Luckily, the distance between her and Tazuna after being near the unconscious boys was minuscule, but she didn't know if she could choose between her friends and Tazuna if she had to escape with anyone in the case that Kakashi failed.
"You did a number on my genin, I can't exactly just let you leave scot-free," Kakashi jived.
"Are you sure it's…" Zabuza wondered, "Time for jokes?" Kakashi and Zabuza queried together.
"This again," Zabuza complained only for Kakashi to comment at the same time as him.
"Ninpo, Hidden Mist Technique," they both called forth.
Kakashi had conserved the majority of his chakra, so in this situation, his mist overpowered Zabuza's, who'd been nonstop firing jutsu and losing chakra when being tagged by Lu. If not for the genin's planning skills and Lu's kekkei genkai, Kakashi guessed he would've barely survived with his students' lives and his client somewhat unharmed, even worse, he could've died sealing all six of their fates. Kakashi was about to sneak attack Zabuza with a raikiri from deep in the mist after distracting him, but before he could get near Zabuza, the man dropped down, his mist vanished, and Kakashi could barely see a hunter-nin confirming the Demon of the Mist's body as dead, taking it, and body flickering away.
Author's Note: I've decided to use both "jutsu" and "technique" when describing jutsu and Japanese translations for certain jutsu and items and English ones. I hope it doesn't come off as too inconsistent.