As the days passed inside the dimension, Gray noticed Izuku actually getting used to the grueling conditioning workouts from Master Roshi. His endurance was climbing. Gray decided it was finally time to fix the kid's terrible habit of using One For All. In the original show, Izuku constantly broke his arms, legs, and fingers every single time he tapped into the power, up until he finally trained under Gran Torino and figured out Full Cowl. Gray planned to fast-track that realization right now so the kid could avoid shattering his bones ever again.
Once the training for the day wrapped up, a completely exhausted Izuku walked back to the residential housing block Gray had made appear. The second he lay down on a mattress to let his aching muscles rest, he heard a knock. He turned his head and saw Gray standing in the doorway.
"Come follow me," Gray said simply.
Izuku groaned miserably but forced himself to stand up. He really didn't want to move a single inch for the rest of the night, but Gray was the one helping him get stronger, so he complied and followed behind. Besides, after accidentally peeking at Gray's solo training earlier that week, Izuku knew he absolutely did not want to get on his bad side. He had literally watched Gray casually throw a small, glowing ball of energy toward a distant mountain range and saw the entire peak get completely vaporized in an instant.
Once they walked down to the beach near Kame House, Gray grabbed All Might to help demonstrate the lesson.
"All Might, if you can, please transform into your muscle form and throw a one-hundred-percent punch into the water," Gray requested.
All Might didn't question the instruction. He immediately bulked up into his towering form and threw a roaring fist straight at the ocean. The resulting air pressure from his knuckles made the sea violently explode outward.
"Now, Izuku, I want you to do the same exact thing," Gray said, turning to the boy.
Izuku stopped dead in his tracks. He stared at his own fist, knowing exactly what was about to happen to his body. But not wanting to say no, he reluctantly stepped up to the shoreline, ignited One For All, and threw a maximum-force punch. The power surged wildly, completely destroying and shattering his entire arm in a sickening snap.
Gray didn't even blink. He reached into his pocket, grabbed a green Senzu Bean, and popped it into Izuku's mouth. "Eat it."
Izuku chewed and swallowed. The broken bones and torn muscles instantly healed over, returning to normal before his eyes.
"If you try to copy All Might's current output, you will permanently destroy your body, correct?" Gray asked directly.
"Yeah," Midoriya nodded, holding his fully restored arm and looking down at the sand. "I'm... I'm still not strong enough to handle this power."
"Then why the hell are you still trying to copy him?" Gray said, pointing a thumb back at the Symbol of Peace. "You have legs you can kick with. Or, you know, you could just choose not to use one hundred percent of the power."
Izuku's brain completely short-circuited. He blinked blankly. "What... what do you mean?"
Gray let out a heavy sigh, crossing his arms. "Listen, I know he's your idol and whatnot, but you don't have to be another exact clone of All Might. If your body can't handle one-hundred percent, why not try fifty percent? Hell, why don't you try to use just one percent and see where that takes you?"
Izuku began murmuring frantically to himself, processing the advice. He closed his eyes and attempted to execute the thought, mentally squeezing the massive river of energy inside his body down to a tiny trickle. He focused the energy entirely into his right fist, trying to make the output as low as humanly possible.
'One percent... SMASH!' Izuku shouted inside his mind, throwing his fist forward.
A small, distinct gust of wind popped in front of his knuckles, and he didn't feel any pain or broken bones at all. Feeling incredibly happy with his success, Izuku turned around to smile at Gray, only to be met with a sudden, incoming punch.
Izuku couldn't react or dodge fast enough. Oof!
Gray's fist connected lightly with his stomach, making him instantly hunch over and gasp for air. All Might panicked, instinctively stepping forward to intervene, but a firm hand clamped down on his shoulder. It was Aizawa. After spending a few days dealing with Gray's unorthodox training methods, the underground hero knew the stranger always did things with a calculated plan in mind.
"Let's just observe for now," Aizawa told All Might, keeping his eyes on the pair.
"Do you know why you just got hit?" Gray asked, looking down at the groaning boy.
Izuku, still hunched over, clutching his gut, managed to squeeze out a reply. "No..."
"Okay. Do it again," Gray ordered.
Izuku took a painful minute to catch his breath, stood back up, and attempted to throw another focused 1% arm strike. The exact second his punch finished, he was immediately met with another quick tap to the stomach from Gray, sending him back to his knees.
"Why..." Izuku wheezed, looking up in frustration.
"I thought I just told you to stop trying to be All Might?" Gray stared down at him, his expression serious. "So why do you keep trying to activate the quirk as if you were him?"
Gray stepped back to give a physical demonstration, throwing a fluid mock punch into the empty air. "See, the thing you're doing wrong is activating the power only when you want to throw a specific strike. You make the power appear exclusively on your arm when you swing, correct?"
Izuku nodded slowly, listening intently.
Gray then pointed a finger directly at the boy's chest. "That is your core issue. Do you only throw a proper punch using your arm? No, right? You use your back, your waist, your legs, and your entire body's momentum to drive the force forward. But you're only activating the power in a single limb. Why don't you try to channel that low percentage all over your entire body at once? That way, you can actually move, dodge, and react to incoming counter-punches the second after you throw a strike."
Something massive seemed to click inside Midoriya's mind, his eyes widening as the concept of full-body distribution.
"Go on, try it," Gray told him.
Izuku took a deep breath. He closed his eyes. Instead of forcing all the energy into his arm, he spread the 1% power evenly throughout his entire body. Green sparks of static electricity started dancing around his skin. He successfully achieved the full-body distribution, but his muscles locked up tight. He felt as if it were almost impossible to move.
"Try to keep it going for as long as you can," Gray instructed, stepping back into a loose stance. "Okay, here it goes."
Gray drove a light punch straight toward Izuku's stomach. Izuku focused on his reflexes and managed to awkwardly sway his torso out of the way. However, the sudden movement broke his intense concentration. The green sparks instantly vanished. Gray, noticing the power drop, immediately threw a follow-up punch that landed squarely against his ribs, sending Izuku right back to his knees.
"You have to keep it active at all times... otherwise, it will not work," Gray said firmly. "Get up."
Izuku pushed himself off the dirt, gritting his teeth. He stood up and attempted the full-body distribution again. They continued this grueling routine for hours under the night sky. Gray kept throwing light, predictable punches, forcing Izuku to maintain the energy flow while dodging under pressure. By the time the sun started rising, Izuku finally had the hang of keeping the green sparks active while moving his limbs.
"Good," Gray said, nodding in approval. "Now try to slowly up the percentage and find out your current body limit."
Izuku concentrated harder, letting more of the reservoir flow into his body. The green lightning around his body grew brighter. He pushed it up to 2%, then 3%, until he finally hit 4%.
"This is my limit... four percent," Izuku squeezed out, his jaw clenched tightly as his muscles vibrated from the pressure.
"Good," Gray continued, patting the boy's shoulder. "Tomorrow, I want you to do your entire conditioning training with your four percent active at all times. Get your body used to the constant weight of the power."
"Okay. Thanks," Izuku panted, turning off the energy.
Thanks to the Senzu Bean he had eaten earlier, his physical injuries were perfectly healed, and his stamina was fully restored. Seeing that the kid was fresh, Gray didn't let him slack off. He sent him right back to continue his late-night physical drills with Master Roshi for the rest of the evening.
"Once you get used to the baseline weight of your training clothes and your four-percent output, come and find me," Gray called out as Izuku walked away. "I'll make your weights heavier."
Izuku looked back with an expression as if someone had just killed his favorite puppy, but he nodded along anyway. It hadn't even been a single week since they stepped inside the Training Dimension, and he had already made massive steps toward mastering One For All without breaking a single bone.
Gray used the downtime to walk over to the opposite side of the mountain range to check on Mirko's progress. Apparently, the wild Rabbit Hero had boldly challenged all three of her legendary instructors, Garp, Zephyr, and Rob Lucci, to a fight. Predictably, she was losing over and over and over again. Yet, that wicked, battle-hungry smile of hers never left her face. Even when getting beaten completely to a pulp by Garp's fists or Lucci's precise strikes, she trained with absolute seriousness. During her designated resting time, she would just stand right back up and challenge them again.
Gray smiled, watching her trade high-speed kicks with Rob Lucci. He knew that if she successfully mastered the six Rokushiki techniques, her combat efficiency would skyrocket to a point where she could easily contend for the Number 1 spot in the official hero rankings. And since her fighting style already heavily focused on powerful leg strikes, three of the six powers, specifically Geppo (Moonwalk), Soru (Shave), and Rankyaku (Storm Leg), should become natural for her to learn.
Leaving the martial artists behind, Gray walked back to his isolated mountain sector to continue his own personal grind. He decided to attempt to break through the ultimate barrier and turn into a Super Saiyan.
Gray dropped into a wide, solid stance. He closed his eyes and began pushing his baseline power level higher and higher, forcing his Ki to expand.
"HAAAAAAAAAAAAA!" Gray yelled at the top of his lungs.
He focused his mind, trying to locate and trigger that specific "tingle" sensation in the upper-middle back that the Universe 6 Saiyans had described in the show. But no matter how much energy he channeled into his spine, he couldn't get the transformation to spark.
Even without the transformation triggering, the sheer density of his expanding Ki made the entire simulated mountain range vibrate and shake violently. Miles away, the pro heroes and Midoriya stopped what they were doing, feeling the massive pressure wave wash over their skin in awe.
All Might stood on the beach, staring toward the shaking peaks with a deeply serious expression. He had been fully aware that Gray had heavily held back during their public fight in the mountains. But after witnessing the terrifying caliber of Gray's daily training regimen and the monstrous, simulated enemies he kept fighting in the distance, All Might realized the truth. If this person ever decided to turn evil, absolutely no one on the planet would possess the power to stop him. He let out a quiet sigh of relief, deeply glad that Gray had chosen to stand on the side of the heroes.
After two weeks of being inside the training dimension, Gray checked in on Izuku. The kid was moving smoothly under the pressure, so Gray used his clothes beam to make Midoriya's weighted clothes even heavier, pushing his 4% Full Cowl limits.
With Izuku's baseline conditioning well underway, Gray decided it was time to shift Aizawa's focus. He pulled the underground hero away from Master Roshi's conditioning training and led him to a completely different side of the dimension. With a thought, Gray willed a massive, dense city layout to manifest out of the void. Right in the center of the urban landscape, he spawned Bang and Bomb from One Punch Man to train Aizawa in the Water Stream Rock Smashing Fist martial arts.
Gray knew from his knowledge of the show that Aizawa was incredible at handling himself against crowds of regular villains, just like he would eventually prove during the USJ attack. However, the moment he fought someone naturally stronger, more durable, or faster than him, his lack of high-tier hand-to-hand combat skills became a liability, and he would lose. Putting him under Bang's legendary, defensive-redirecting martial arts teachings would completely eliminate that weakness.
The only person inside the dimension who hadn't received a specific training regimen was All Might. Gray chose to leave him be. Since the Symbol of Peace had already passed the core of his quirk onto Midoriya, forcing him to do intense physical workouts would be completely pointless. It would only make his muscle-form time even shorter than it already was, which was the last thing they needed. All Might simply stayed on the sidelines, acting as an advisor and watching the next generation surpass expectations.