Both Kakashi and Guy rushed towards Kakuzu.
But just as they were about to destroy Kakuzu's remaining hearts, Kakuzu broke free from Yamanaka mind takeover and moved.
Guy still hit him, though not exactly at his heart.
But Kakuzu hadn't realized that his last heart mask had been under Shadow Possession after getting his mind taken over.
Chidori hit the heart mask, and now Kakuzu was now down to his primary heart. So he started moving it around in his body.
And he was pretty injured as well aside from the hearts, due to tanking paper bombs and Guy.
'I need to get hearts—'
But Kakashi and Guy weren't giving him the time. They were pressing him.
"It's showtime of Youth! 6th Gate! Open!"
Guy opened the most gates he can open right now.
'1st Gate! Open!'
Kakashi also opened the only gate he can. He needs more chakra to finish Kakuzu off before he can try to take the heart of one of his teammates. And he has a ninjutsu to use.
Despite difficulties, Kakuzu was able to somewhat keep up with Guy and Kakashi. The problem was that he wasn't able to form handsigns.
"Don't underestimate—"
Kakuzu was about to shout, but his touch skin near his heart was previously broke to Guy's kick.
'Now's my chance!'
With his Sharingan, Kakashi has obviously realised that Kakuzu is moving his main source of chakra, his heart, around his body very quickly.
Meaning if he misses his final Chidori, he will loose his precious chakra. And his hand will be stuck in Kakuzu's black threads, which will be dangerous.
What he needs right now is not the precise penetration of Chidori, but sheer destruction of Rasengan.
More than Rasengan to properly finish him off.
So, he instantly formed a Rasengan with his right hand, and added Lightning nature chakra with his left.
Even if it will not be perfect, it will increase the destructive power a lot. And the Lightning should be able to travel throughout his thread and paralyse his heart. Even if it doesn't work fully, Kakuzu will get a heart attack!
Without going full power on the lightning nature chakra, he smashed it inside the broken skin.
'Lightning Rasengan!'
"Guy! Retreat!"
Both Guy and Kakashi jumped away at the last second, and Kakuzu wasn't sent flying even a little bit as he would be with a Normal Rasengan.
"AAAAAAAAHHHHHHHH!"
Kakuzu screamed as the Rasengan was lodged inside, it's lightning increasing, spinning Kakuzu's insides.
And then, it finally expanded. A brilliant blue thunder swirl. With sparks of lighting. Though not perfect, but quite destructive.
Kakashi noticed that his headband and weapons were somewhat attracted to it.
'Electromagnetism?'
Dust settled, and Kakuzu laid dead, his entire body, especially the black threads tattered.
And Sharingan confirmed that Kakuzu is dead.
Kakashi sighed in relief, lowered his headband and sealed Sharingan back with handsigns, and deactivated the First gate.
He immediately went limp and fell on the ground.
'I wonder how Guy handles the strain of higher gates.'
"Kakashi! That was a Youthful Tag Team showdown—Ack!"
Guy deactivated the gates, and he fell down as well. His whole body twitching, just like Kakashi.
"Yesh."
Kakashi smiled beneath his mask, relishing in the fun of his first actual battle.
He couldn't take Kakuzu 1v1, obviously. And even with Guy, the Yamanaka, and Nara provided great support.
And most importantly, without his prior knowledge of Kakuzu's money hungry nature, and packing a lot of stock of paper bombs of Kushina because he was paranoid about his first mission, he wouldn't have been able to take out two of Kakuzu's hearts and injure him before the fight even started.
And he got the idea of adding chakra nature in Rasengan with another hand from canon Naruto.
Without all this, defeating Kakuzu would have been a pipe dream.
This made him painfully aware that he needs to become much stronger from where he is standing right now.
Afterall, Kakuzu is merely the second weakest member of Akatsuki.
Still... fighting felt great.