Madara Would be Proud

"Get him!"

Faced with a sea of pink-haired girls, Kakuzu instinctively took half a step back— even though his opponent had divided her chakra into a thousand different parts, her Shadow Clones didn't seem any weaker than the original! And to make matters worse, he had lost track of the original…

Nevertheless, Kakuzu had seen much worse during his long career as a shinobi— it would take more than this to deter him from taking Sakura's head and the hefty bounty that came along with it!

"Fine then, if that's how you want it," said Kakuzu, his fingers blurring as he chained hand signs one after the other in quick succession. "Pest removal it is…"

Kakuzu clasped his hands together, and his four masked dolls jerked into motion, their masks' mouthparts clattering eerily. The Lightning-mask doll skittered to the Water-mask doll, while Fire-mask doll lumbered over heavily to the Wind-mask doll. Their threads reached out to each other, and the two pairs of roiling masses of black strings began melding with each other…

"As if we'd stand by and watch while you transform!" Sakura's Shadow Clones shouted. "Shannarooo!"

"Earth Style: Mud Wall!"

The ground in front of Kakuzu shifted, and an earthen rampart twenty metres wide emerged from the ground, causing the Shadow Clones' various Fire and Wind Style Ninjutsu to explode harmlessly against the chakra infused mud. The shearing winds failed to inflict any lasting damage, and the fireballs only served to bake the wall's clay, actually reinforcing its defensive properties!

 'Now's the time,' Inner Sakura said urgently. 'While he's not looking!'

The purpose behind all these setups— using the Breast Diet Jutsu to sacrifice her bustline in exchange for chakra, then using this chakra to complete her Yin Seal, just to unlock the Breast Seal Fūinjutsu that the System had deduced for her— it had all been for this very moment!

In the original timeline, the Rasengan's inventor, Namikaze Minato, had failed to take his signature Jutsu to the S-rank level. He had attained the pinnacle of chakra shape transformation, but he hadn't succeeded in incorporating chakra nature transformation into his Jutsu.

The reason for his failure was the difficulty that lied in multitasking; juggling both shape and nature transformation at once. As Kakashi had ended up pointing out, it was like trying to look left and right at the same time.

In the end, it would be his son Naruto who would end up solving this conundrum by simply adding more Shadow Clones to divide the burden:

His original self to provide the chakra in his palm, a first Shadow Clone to perform the shape transformation (rotation and compression), and a second Shadow Clone to perform the nature transformation outside of his body— giving birth to the S-rank Ninjutsu Rasenshuriken.

Naturally, performing the Rasenshuriken would be much simpler if one could perform the nature transformation internally (since manipulating chakra outside of one's body is magnitudes higher in difficulty), and then shape the chakra, but there are a few reasons why this is ordinarily impossible.

Firstly, chakra that has been endowed with one of the five natures is much more corrosive to the human body compared to chakra that is attributeless.

As a reminder, humanity did not evolve to obtain chakra or come by chakra through natural means— channelling ordinary chakra is already quite irritating to one's chakra pathway system, so one could imagine just how much harsher it is to channel lightning or fire. As such, it's in one's best interests to expel elemental chakra from one's body in the form of Ninjutsu as fast as possible.

Secondly, one must bear in mind that both the Rasengan and the Rasenshuriken do not use hand signs.

Most Jutsu generate a lump of chakra and then release it in one burst; if there's any chakra left over, it goes to waste and dissipates. However, the Rasengan and its variants require one to gradually accumulate chakra in the palm of one's hand and at the same time shape it into a vortex— and therein lies the rub.

If you concentrate on shaping, you can't concentrate on performing nature transformation. At the same time, it's impossible to store the vast quantities of elemental chakra necessary to form a full-fledged Rasenshuriken ahead of time— the chakra can't be released all at once, as it needs to be carefully shaped; even if the process is done relatively fast, from an outsider's point of view.

However…

What if the elemental chakra could be stored ahead of time? Like, say… within the Breast Seal Fūinjutsu? Then, all Sakura would need to worry about would be shaping the chakra that would have already been subjected to nature transformation!

Naturally, the final step of any Rasengan variant (containment) would still require her to envelop her Rasenshuriken with attributeless chakra, so that she may actually hold the damned thing, but all in all, this version of the Jutsu would require no Shadow Clones. This meant two less points of failure, making Sakura's Rasenshuriken much more reliable than Naruto's!

And for her Rasenshuriken's nature, Sakura naturally chose fire. Wind was a known quantity, since she knew that Naruto's Wind Style Rasenshuriken worked with absolute certainty, but her affinity towards fire was simply too high not to use. In Sakura's hands, her blue flames seemed almost alive….

B-cup…

'Breast Seal at full capacity!' Inner Sakura announced. 'It's now or never!'

Sakura glanced over at the battlefield, where her Shadow Clones were now single-mindedly keeping Kakuzu occupied by climbing over the mud wall and launching kamikaze attacks at him and his dolls— and in response, Kakuzu pierced them, choked them, threw them into each other using his extendable arms…

They had done their jobs, so Sakura dispelled her Shadow Clone Jutsu, saving only a single one, just in case.

Memories, fatigue and pain flooded into Sakura's mind, sending her reeling, but it had to be done— she couldn't afford having a Shadow Clone dying at the wrong moment, which might distract her and cause her Jutsu to explode in her face.

!!!

It felt as though her veins had filled with lava, but Sakura endured through the pain, sending the sealed fire chakra from her right breast to her right hand and spinning it into an orb of rapidly rotating azure flames. At the same time, she held her left hand over the orb, compressing the fire attributed chakra beneath layers and layers of ordinary blue chakra, but the containment field was only serving to make the Rasenshuriken's dense core burn even hotter….

"Fire Style: Rasenshuriken!"

Exhausted, Sakura held up the blue jewel in her hand. Once she threw this, all hell would break loose…

'Hang on,' Inner Sakura pointed out. 'How exactly are we supposed to throw this!? Don't we need Sage Mode for that kind of chakra control so far from our body?'

Sakura looked down in horror; she had a literal bomb in her hands! She hadn't thought this far ahead— the moment she let go, the Rasenshuriken's containment field would unravel, and it would explode!

"It's all yours," said Sakura, placing the pulsating ball of luminous blue flames in her last Shadow Clone's hands. "Go get 'im."

"Thanks," said the Shadow Clone glumly. "Will do…"

The moment the bomb was out of her hands, Sakura turned on her heel and ran away, waves of exhaustion washing over her as her chest finally turned as flat as a washboard. Without any hesitation, Sakura activated her Yin Seal and powered through the fatigue.

From the sounds of roaring flames and crackling lightning behind her, Sakura was vaguely aware that Kakuzu's masked string dolls had launched in pursuit after her, and then…

A flash of intense blue light flared up behind her, followed shortly by a blast of scorching hot air, which blasted her off her feet. Sakura fell, and felt something hitting her head— the colour in the world went from blue to black, and then she knew nothing.