A/N: No-one is going to shout out the name of their jutsus because it's not very smart to give your opponent forewarning, but I will be putting the name of the techniques in footnotes :) Kakashi is going to be strong and smart but not completely OP :)
Mantis has a fearsome sort of determination to her as she whips out ninjutsu after ninjutsu, sending anything from Katons to Suitons hurtling towards Kakashi with frightening accuracy. Kakashi flits across the floor in a blur, shunshin quickening every movement as he leaps, twists, and flips with the agility of a cat.
As Kakashi escapes another crashing wave of water – voluminous despite being drawn from thin air – she shunshins through the spray to come up behind the teen. Mask tilted up fraction, she spits out a senbon while throwing a kunai simultaneously. Sakumo's insides twist hard as Kakashi jerks away from the kunai, only to put himself in the way of the senbon, the slim length of steel perfectly positioned to hit the tenketsu at the nape of his neck.
The edge of the kunai opens up a line of red but the senbon bounces off the metal plate of the gloved hand Kakashi cups behind his neck. With a flick of his other hand as he turns, Kakashi sends several kunai flying in retaliation, each tied to a length of near invisible wire, forcing Mantis to backflip away. The wires pull taut midway, changing the kunais' trajectory into a sudden sideways swing, making Mantis' eyes widen behind her mask. She raises her left arm, the armoured plates along her forearm managing to block most of the kunais but two slip past her defences to carve long grooves across her ceramic mask.
Mantis slides to a stop, her feet digging grooves into the wet earth as she adjusts her mask back into position. "Ouch," she complains, her voice lilted with humour. "You shouldn't aim for a lady's face."
Kakashi's hands twitches restlessly where they are positioned before his chest, ready to fly into a jutsu. At his neck, a crimson bead of blood wells within the tear in his mask before it disappears, soaked into the fabric. "If only you stop aiming for my neck," Kakashi huffs.
Mantis tilts her head in acknowledgement. "Touché."
Then they're clashing again, Kakashi turning on the offensive this time, having observed Mantis for long enough to work out the pattern of her movements. He sends out several waves of shuriken to force Mantis towards a corner – the audience has to dodge – and his hands blur into seals.
His form doubles and splits into two, each running in the opposite direction as a blinding length of lightning pulls between their extended hands*. Boxed in, Mantis takes off running up the wall with chakra-sticky feet, both Kakashis following close behind. When she reaches the ceiling, she turns and lobs a several fireballs at the teen in an attempt to counter his Kage Bunshin.
One Kakashi dodges to the side but the other Kakashi drops off the ceiling, plummeting towards the floor below. Rin shrieks and a strangled sound escapes Sakumo but before anyone can do anything, the falling Kakashi disappears just as one of the lights behind Mantis goes out.
A rod of lightning spears along the ceiling, grazing Mantis in the side and sending a smattering of sizzling blood onto the floor below. Cursing, Mantis staggers away, clutching her wounded side where her armour had been seared through.
"Ah... a Kawarimi!" Kagami exclaims. Sure enough, there are two Kakashis sticking to the ceiling again, one in front of Mantis and one behind, where the light had gone out. A length of crackling lightning stretches between their hands.
The switched-out light bulb tumbles down to the ground, as if in slow motion, its glass glinting even as its white-hot tungsten filament cools to a glowing red.
"Time's up," Snake yells from the sidelines and someone shunshins out to catch the light bulb before it smashes to smithereens. Kakashi's lightning fizzles out and the room seems dimmer for it.
"Nice feint." Mantis laughs as she straightens painfully. Kakashi's clone disappears in a fizzle of blue sparks.
"Thank you," he replies. "Are you alright?"
The ANBU waves his worries away and they begin walking down the walls. "I think you cauterised it."
"Medic~" Snake calls loudly and Mantis rolls her eyes.
Once they've reached the floor again, Kakashi and Mantis hook their fingers together in the sign of reconciliation. It's only when Snake starts fussing over them that Sakumo finally feels like he can breathe properly again. Kakashi is a capable warrior - more than capable - but there's a part of Sakumo that will always be worried about his boy.
If only Kakashi would stop plummeting off two-floor-high ceilings and spare his father a heart attack.
"Kakashi is awesome isn't he?" Rin cheers, having recovered her excitement. "He's going to beat all of them!"
'Of course he is,' Sakumo thinks fondly, watching Kakashi sneak the mouth of a water bottle behind his mask.
*Raiton – Kage Bunshin no Jutsu; Raiton – Raiden
A/N: Did I manage to explain what Kakashi did properly? Basically, Kakashi and his clone chase Mantis up to the ceiling and when she throws the fireballs, Kakashi dodges while his clone falls off the ceiling. Halfway through the fall, the clone Kawarimis with the light bulb of one of the ceiling lights behind Mantis. Then the Kakashis do a Raiden and when the lightning forms between them, it pierces Mantis' side.
Omake
"One Thousand Years of Death!" Kakashi declares as his second opponent flies into the air screaming, his hands clenched over his bum.
"That's my boy!" Sakumo shouts, springing to his feet and punching the air. Rin and Kagami faint dead away.
(In Ep4 of Lee's Springtime of Youth, a foreign ninja said he was attacked by Sakumo with that "jutsu". Can you imagine Sakumo teaching it to tiny Kakashi?! And if anyone asks Kakashi what he does when he misses his father, Kakashi will just smirk and say "let me show you", and then One Thousand Years of Death them.)