The Land of Waves I.
They had to travel really far away on foot, and even worse, with a walking pace, because Tazuna was just a normal human. It wasn't all that boring though. They were attacked the same day they set out by the Demon Brothers, or something similar.
Kakashi, due to his rank, was attacked first. The two chuunin-level ninjas from Kirigakure wanted to take out the most troublesome first, then move on to the kids. Kakashi didn't resist, and was 'killed'. Both of them moved towards Dix next, but Sasuke moved at the same time. They were intercepted, and Sasuke managed to fight them to an extent. Dix just waited with crossed arms.
Next the Demon Brothers split in two, one going for the real target while the other tried his luck with Dix once again. The assailant was rushing towards him as fast as possible, so all he had to do is sidestep at the last second and let him pass, then throw a kunai into the back of his head. The enemy was killed within a heartbeat.
All this while the 'dead' Kakashi rushed in to neutralize the second attacker that was moving towards Tazuna and the two other kids. It was a clean and fast fight, expect for Kakashi's trick at the start. He checked around to see everyone was alright.
"Sorry for not helping right away, but I knew you would handle yourself well. Good job, Sasuke, Sakura. I see you made quick work of your share, Naruto... Let's go, we have much to do."
Kakashi then spoke with Tazuna about him lying about the mission. The team was supposed to protect the client from petty thieves or bandits, but not from ninjas. That would make the mission at least a B-rank and much more expensive. Tazuna was simply broke though, and could only pay for a D-rank mission.
They had the option to turn back or continue. Sakura voted to abort the mission, while both Dix and Sasuke said they want to continue. Kakashi was also pretty confident, so he agreed and they continued.
Nothing much happened until they actually arrived to the Land of Waves trough a small boat that carried them over the huge masses of water that surrounded the location. Not long after they hit shore they had to move trough a forest to get to their destination. Dix knew they will be ambushed there.
After a few minutes of walking Dix sensed a 'strong' chakra signature's arrival. It was waiting in ambush a few hundred meters up ahead. Dix didn't bother telling the others, he just walked in the front. When they arrived to the ambush position Kakashi finally noticed the attacker too and shouted:
"Get on the ground!"
The huge Beheading Sword was flying towards Tazuna, the target of the assassination. Everyone dropped except Dix. He just jumped towards the sword and caught it mid-air. He checked out the weapon, it looked really nice, it was actually almost 1,5 times bigger then him. Next he held it up and started waving it as he shouted out.
"Hey man! You dropped something!"
"Hahahahaha! Kid, you are good, and funny at that!" came the deep voice from the shadows of many trees at once. "Kakashi Hatake, the infamous sharingan user, Copy Ninja of Konoha, you found yourself an amusing little wingman." continued the voice, then its owner suddenly appeared on the branch of one of the trees. Sakura gasped, while Kakashi identified the attacker.
"Zabuza Momochi... Elite rouge ninja from Kirigakure. So they sent someone of your caliber next. The client must be pretty important." he said, then removed the special headband from his left eye to become completely combat-ready.
"Haha, yes. I'll have to take that old man off of your hands. But the first target will have to be you, Kakashi."
A barely noticeable mist started gathering around them. It was getting thicker and thicker, then Zabuza jumped on the nearby mass of water. At this Kakashi gave his instructions to the team.
"Guys, you can't fight this enemy, you will only be in the way. Get into Menji battle formation and protect Tazuna. The teamwork now is that you don't get involved in this fight."
Everyone got into position, even Dix, and they waited for Zabuza to move. He released the Hidden Mist Jutsu while standing on the water, which made the mist around them so thick it was impossible to see even those close to you.
Zabuza's exact location at this point was unknown, even though Kakashi blew away the mist in a few seconds with his chakra. Dix knew he will be targeted first, as he still had Zabuza's sword with him, and that idiot only had one weapon, so he needed that. He was right, at the exact second the mist was lifted Zabuza compromised their formation and appeared between them.
"I'll take back what's mine, kid!" he shouted, but Dix welcomed him with a kunai, even though Zabuza was behind him. He used the strength of his fingers and threw the kunai over his shoulder without looking back.
"You shouldn't have thrown it like a total retard. This is your only weapon and you won't even think twice of letting it go? What did they teach you in Kirigakure? No wonder the old system broke up." Dix said, taunting his enemy. The cheeky attitude in his voice reached Zabuza and hurt his pride, but not as much as the events of the next few seconds will.
The Zabuza that was behind Dix was actually a water clone, so he didn't bother with that. Next he threw another kunai in front of him to the left, where another clone was hiding, then rushed at the real one. With only these in the anime Zabuza was able to outplay Kakashi and trap him, but Dix didn't really have the patience to wait for the original events to happen, so he opted to take care of the enemy himself.
"Naruto, don't do it!" came the shout from Kakashi, but he was completely ignored.
Dix slashed at Zabuza's position, who was taken by surprised that some shitty genin was able to find him. Dix summoned a few clones while the mist covered everyone, and those multiplied by a bit and attacked Zabuza from many directions, some with projectiles, some in close combat, while the original used the sword to injure his enemy.
Zabuza was quickly overwhelmed, as all his previous plans failed and was caught off-guard. He blocked some attacks coming his way, but most still got trough his defenses, even more easily, because he foolishly got rid of his weapon before. Swords are usually only seen as a tool of aggressive offense, but they are much more than that. The defensive capabilities they provide their wielder is way more important. A true swordsman would never get rid of their weapon in battle.
Kakashi rushed to join the fight, but Zabuza quickly jumped away, with quite a bit of damage on his body.
"What the hell, what are you, kid? How is this possible? I was part of the Kirigakure ANBU Black Ops, how can a mere kid hand my ass to me like this?! I..." he said, but at the end he was interrupted by an attack from an unknown source. The next moment two big senbons, metal needles penetrated Zabuza's neck. He dropped on the ground, seemingly dead.
Everyone was surprised at these events, even Dix himself, because this was supposed to happen around 2 more episodes worth of events later. He never imagined he would be able to speedrun this so much. But it doesn't matter, at least the cogs are turning smoothly.
Soon a small masked ninja appeared on one of the branches, the one who threw the needles. Even Kakashi gasped at all the rushing events, he wasn't even able to throw a punch towards Zabuza and it was all over already. He rushed to the body to check the vitals. He got the impression that Zabuza really died in half a minute of combat. This is certainly crazy...
"Seeing that mask, you must be a Kirigakure tracker ninja..." he said, turning away from the corpse.
"Yes, you are quite knowledgeable. I have been waiting for a long time now to kill Zabuza. You made it seem easy." he said with his young voice to Kakashi, then turned to Dix for the last sentence.
"Well I suppose this is it, and you will take the body with yourself." replied Kakashi as Dix just glared at the new 'face'.
"Yes, that would be for the best. Much appreciated." the boy said then bowed a bit.
Dix knew whats going on, and for a few seconds he humored himself to have another round with Zabuza, but he'd rather take care of the matter right away. Who knows if the master assassin Zabuza Momochi would have the galls to come back and try again after such an overwhelming defeat. He would be very wise if he just ran away, actually. Something he might grow the brains to do while lying on his bed, injured, like a beaten dog.
Dix walked up to Kakashi who was still standing over the corpse.
"Kakashi sensei, even though he seems to be the same age as us, you shouldn't make a parallel. I know what you think, and its right, this is fishy." he said, then turned to the masked kid.
"So you are a tracker ninja right? You only need the body, right? Then what would you say if I were to make sure that Zabuza really is dead, and would, maybe, chop his head off?" he said and planted the huge Beheading Sword, that was resting on his shoulder ever since the masked kid appeared, into the ground, right next to Zabuza's neck. Hearing this the masked ninja froze for a few seconds, then tried to explain it somehow with less self-confidence in his voice than before.
"I... Kirigakure would be sure to not leave this matter rest if you were to damage the corpse that way. I used senbons for a reason. It would be better for everyone if you let me walk away with the body..." he said. At this Dix laughed a bit, then crouched down next to the body. The kid was obviously sweating bullets.
*Jeez, I am being kinda cruel now... Their backstory is quite touching. But it doesn't really matter at this point. These people are cold-hearted killers and will stay that way all the way 'till they die. I don't see any way out for them. I can't say anything to them that will change their minds. There is nothing I can do.*
"I'm sorry, but I learned a bit about anatomy before, and I know that these points you hit, " he said while pointing at the senbons in Zabuza's neck, " are housing special acupuncture nodes that would stop one's heart without killing them. I have no idea why a tracker ninja from Kirigakure would need a rouge ninja like Zabuza alive, but maybe it's just a misunderstanding. You are quite young, maybe it was just a mistake. Let me help you." he said, then with a swift movement beheaded Zabuza with his own sword. "Now its good, you can take him if you want to. I'll keep the sword though."