As Ren was moving forward, he heard the voice of Zabuza that said "Are you sure that you want to fight me brat? If you are as good as that other guy then you have no chance against me" and Ren drawing his sword said "Do not worry Zabuza-san, I'm strong enough to fight you..."
Then Ren disappeared with a Shunpo and appeared behind Zabuza swinging his sword in a horizontal arc, but even if surprised Zabuza was able to parry the attack and counter with a big vertical slash that Ren evade bt sidestepping and once again attack Zabuza by thrusting his sword aiming at Zabuza's neck, who in turn evade by tilting his neck to the left and kicking Ren to create more space between them.
Then Zabuza notice the light cut on his neck and after wiping it clean with his hand he says "Not bad bart, As far as swordmaship goes you are almost better than me, and that is saying a lot considering your age...." then he raise Kubikiribōchō with one hand in a Jodan stance while with his other one he does a ram hand seal and says "...But I'm not only a swordsman, I am also a shinobi!"
And so Zabuza use the Kirigakure no Jutsu (Hiding in the Mist Technique) and fills the air around them with mist so thick that makes impossible to see for normal ninja, and using the famous Sairento Kiringu (Silent Killing) of Kiri he moves behind Ren and attack him, aiming at his head with the board part of Kubikiribōchō, without making a single sound.
But Ren can still feel Zabuza thanks to his spiritual sense, so he duck to evade the blow, and with Senbonzakura's sheath hits Zabuza in the stomach with a side swing, with so much strength that he almost break his ribs, and sends Zabuza flying a few meters.
Then Ren points his open palm at the ground and says "Hadō #58. Tenran (Vigorous Tempest)" and creates a widening tornado-like blast, which is fired toward the grounds, clearing away all the mist and showing the form of Zabuza who just finished doing another set of hand seals, and after kneading chakra in his stomach, Zabuza expels a large quantity of water in the form of a powerful torrent aimed towards Ren, who in turn his forced to evade by using another Shunpo.
Zabuza's Suiton: Suidan no Jutsu (Water Release: Water Bullet Technique) is so powerful that creates a trench in the ground all over its trajectory, the he says to Ren "Ohh, you are pretty fast, but is that all you can do? Because if it is so, then it is better if you give up or you will get hurt as I'm getting serious now" and so Zabuza use the Mizu Bunshin no Jutsu (Water Clone Technique) and creates another 3 copies of himself, who as soon as they are made gets in position and surrounds Ren in a square formation.
Then the four Zabuza stars using a weaker version of Suiton: Suidan no Jutsu, but in rapid succession, forcing Ren to constantly evading, but what Ren didn't notice was that while he was evading all the attacks a great amount of Water was forming in his surroundings, after a few more seconds the attacks stops.
Then the four Zabuza makes another set of hand seals and says at the same time from the four corner """"Suiton: Suiryūdan no Jutsu (Water Release: Water Dragon Bullet Technique)"""" and using the pre-existing bodies of water for this technique, the four Zabuza moulds the water into the shape of four big dragon, which they then direct at a Ren from the four corner.
Seeing that he could not escape this attack with Shunpo, Ren raise his sword in front of his face and says "Yare, yare it's time to get serious too, Chire (Scatter), Senbonzakura" and his Zanpakuto blade scatter in a thousands of small blades that reflect the light in such a way that seem like cherry blossom petals, then all the petal-like blades starts spinning around Ren creating a spinning shield that cover all of Ren's body.
Next the four water dragons clash against the spinning shield but are unable to break it and soon the attack disperse, and then Ren swing his Zanpakuto's handle horizontally and the spinning shield become a long whip-like attack that hit one after another all the Zabuza, causing all the clones to disperse and the original who used Kubikiribōchō to defend himself was send flying through a few trees.
Ren did not give Zabuza a chance to rest and once again by thrusting his handle, he sends another attack at the downed Zabuza who soon gets up and once again swing Kubikiribōchō at the coming torrent of petals while screaming "What the hell is this!!!" but as the petals-like blades and Kubikiribōchō clash, the petals easily shred troughs Kubikiribōchō before passing by Zabuza leaving him with only the bottom half of Kubikiribōchō and thousands of very shallow cuts all around his body.
Then has all the petals still surrounds him he hears Ren voice that says "I think this is a checkmate, in the last attack I could have easily killed you, so is this my win?" then Zabuza looks at the damaged Kubikiribōchō, then at himself, and in the end at all the petals-like blades surrounding him and while sheathing the remaining Kubikiribōchō says "Okay, brat this one is yours, but what the hell is this jutsu? I have never heard of somethings like this before!"
And Ren return Senbonzakura in her sealed form and decide to reveal "Well this isn't a ninjutsu, but it is a Kekkei Genkai, you see this sword, Senbonzakura is a Zanpakuto, a reflection of my soul and power, while this sword makes so that i can't use the regular ninjutsu, it enable me to use some very versatile spell, and as you have seen before it has the special ability of separating into a thousand tiny, slender blade fragments, which fly away from the hilt, leaving only the sword's handle in my hand, and that is only the first level or as I call it the Shikai, the next level is even stronger but I still can't use it"
Then Ren hold out his hand and say "So what do you think are we strong enough to help you? And will you think about join our group?" and Zabuza stood in silence for a few seconds before taking Ren's hand and saying "Yes, brat you are strong enough, and if this matter end well, I and Haku will join your group".