I'm glad you're having fun, but my planet's taking a real beating, here. Pigero, can you make sure that our attacks don't destroy my home?" King Kai asked, realizing the extent of the destruction that his attack had caused.
"No problem." Pigero answered back without hesitation. His Grey Mode, unbeknownst to him, had already evolved into Silver Mode, with a 25X boost to his original power, which was already around 250,000. In other words, his power level was currently at 6.25 million. Stronger than Goku's base form had been during his battle with Frieza.
Meanwhile, King Kai's base power was only at around 15,000. He'd grown a bit since Goku and Raditz had surpassed him, but not as much as he could have, since he didn't work out actively. Currently, he was using the refined version of Kaio-ken times 3, which boosted his power level directly to 60,000, but it was still absolute garbage compared to Pigero. Protecting the planet would be no problem for Pigero at this rate.
"Alright, King Kai! The gloves are coming off now! Let's take this up a notch, alright!" Yamcha shouted, floating into the air to distance himself from the planet before he started his technique. "Kaio Ken times ten!" He shouted.
Yamcha's power level in his base form was 50,000. In other words, while it took a huge toll, using Kaio-ken times ten could boost his power level all the way to 500,000 for a short while.
"You're going to wear yourself out pretty fast, you know." King Kai warned, doing his best to ignore the rippling winds emanating from Yamcha, which were strong enough to tear him to shreds if he didn't focus with all of his might.
"I don't need time. I'm going to throw everything I have into this last attack, King Kai! My entire future as a martial artist, riding on this one last KAAAA MEEE"
"I see. In that case, I'll take my gloves off, too." King Kai sighed, bracing himself mentally. 'Every living thing on this planet, heed my call! Give me a little bit of your energy!' King Kai shouted telepathically, using his spirit bomb technique to gather just a little bit of energy from every single living thing on the planet. It all flowed through him.
"A spirit bomb, huh!? Give me your best shot!" Yamcha shouted, "HAA MEEEE!"
"Not quite!" King Kai shouted, "Spirit King Enhancement! TIMES THIRTY! SPIRIT KING WAVE!!!!!" Using the energy from the spirit bomb, mixing it with the Grey Mode technique, then blasting it out in an insane burst of pure power, King Kai was able to evolve the Grey Mode technique on his own, enhancing his power by thirty times his base 15,000, just with the energy from his own puny planet.
"HAAAA!!!!" Yamcha shot his Kamehameha wave in response to King Kai's attack. The two beams met in the air with power far stronger than King Kai's planet was ready to take, so Pigero covered the planet with his own energy before it could disintegrate into dust as a consequence of the incredible strength that the two were emitting.
"I can't lose this! HYAAAH!!!!" Yamcha shouted, amping his own wave up even more.
"Just like I thought." King Kai's power was slightly weaker. His beam was slowly overtaken. "Pigero, you got it?" He shouted back at the stronger fighter.
"Yup. No problem!" Pigero answered confidently.
"Alright, then." King Kai said breezily, breaking his energy wave off and dodging Yamcha's Kamehameha right before it hit him. The powerful energy wave shot past him toward the planet below, but Pigero already had it covered. "Let's just block it like this." Not even Pigero could take an energy wave with a power level of 500,000 behind it lightly. Even though it was nothing to him, it was still more than powerful enough to erase all of Snake Way and keep going, so the young fighter was forced to intercept the wave by backing his energy wall with his physical body. Still, though, it didn't budge Pigero an inch, nor was a single rock on King Kai's planet disturbed.
"Aargh." When King Kai's power-up ended, the kickback had him down on one knee for a moment.
"Are you okay, King Kai?" Pigero asked, helping King Kai stand up.
"That technique... It's got even more kickback than the original Kaio-ken. Don't use it lightly, Pigero." King Kai warned, getting to his feet and using his shirt to wipe the sweat from his brow.
"Understood, King Kai." It only made sense. You were essentially adding foreign energy to your own, already barely controlled energy. The huge kickback was a natural consequence, but it'd be an incredible fallback option if things really got grim.
"Uhh, King Kai?" Yamcha asked, landing on the planet.
"Oh, Yamcha. Looks like you won. Congradulations. You beat your own teacher in a fair, all out brawl! Not so useless, huh?" King Kai told the Z-fighter, stunning him.
"Wait... whaaa?" Yamcha was surprised, "I really... did it? I beat King Kai himself!!! I beat King Kai in a fight! I'm not a weakling, after all!" He whooped, celebrating. "I beat King Kai! I beat King Kai! Look, monkey! I beat King Kai!" Yamcha grabbed a very confused Bubbles and started swinging him around. Being a monkey, Bubbles just kind of went with it.
"Wow, King Kai. It's been a while since I've seen you put out that kind of power." Gregory said.
"Yeah, I really should start exercising..." King Kai muttered, "Just don't tell Yamcha that the entire point of the fight was to teach Pigero some new techniques."
"You got it, King Kai! My lips are sealed!" Gregory said obediently, saluting for dramatic effect.
"What?" Yamcha asked, overhearing Gregory.
"Nothing!"
"So, Yamcha... you still giving up martial arts?" King Kai asked Yamcha, already knowing the answer.
"Absolutely not! I'll train hard, until one day when I can beat Pigero, too!" Yamcha answered cheerily, punching the air.
"Alright! Then it's time to break for lunch!" King Kai told them.
After lunch, Yamcha immediately took a nap while Pigero continued his training.
Just doing image training, King Kai managed to quintuple his base fighting power, and almost beat Yamcha. It's likely that deep down, King Kai's relieved that he didn't do any real training, because he would have absolutely squashed Yamcha into the ground like a bug (Though he wouldn't have beaten Pigero, who's quickly turning into a monster even more powerful than I'd expected).
In the original, Yamcha didn't learn Kaio-ken, and he had still been strong enough to beat Recoome, so he ended up pretty strong here, but that'll drop to being average by the time he comes back to life.
Yamcha's likely destiny: To be absolutely unremarkable.
But he's not giving up yet.