Now that Ash-Charizard had proven his strength, Ash felt that it was time for Greninja to grow even stronger. He could feel it; they may have mastered the transformation, but they hadn't come close to even unleashing Ash-Greninja's full potential. The power that Ash-Greninja had displayed against Sawyer's Mega Sceptile and Alain's Mega Charizard X were only the tip of the iceberg. He hadn't told any of his Alolan friends about having the Bond Phenomenon, only Professor Kukui and his wife Professor Burnet knew. He had only introduced them to his friends in Alola, nothing more. Charizard had developed quite the rivalry with Kiawe's Turtonator, while Lana was thrilled at seeing Greninja, as was Popplio. They had even proven their strength in the base forms: Greninja had effortlessly tanked Stenee's Magical Leaf before swiftly defeating her with Aerial Ace, while Charizard had endured Turtonator's Shell Trap before delivering a devastating Seismic Toss that knocked him out cold.
What pleased Ash even more was that his Pokemon team were growing stronger by the day. Torracat had mastered not only Flamethrower, but also had learnt Slash and Iron Tail, courtesy of Pikachu the teacher. Lycanroc had been sent away briefly; Ash had Hoopa teleport him to the Gym of his old acquaintance, Grant, the Rock-Type Gym Leader of Kalos, where he had asked Grant to mentor Lycanroc on some new moves. Grant had accepted readily, and less than two weeks later, Lycanroc returned with much greater attack and endurance, and had learnt moves like Thunder Fang, Rock Tomb and Stone Edge. Rowlet had also recently evolved into Dartrix, and had mastered Razor Leaf, Steel Wing and Brave Bird, thanks to Greninja, Bulbasaur and Staraptor, whom Ash had brought over temporarily. All in all, everything was going well.
Now Ash wanted to take his next big step: having a major battle with Greninja again. And he knew just the opponent for the job. "Hoopa! Please take me to this place…"
"Ash?!" Standing in front of the gang was a bipedal, feline creature that was mainly gray in color with a huge purple tail at the back. Ash smiled.
"Hello, Mewtwo."
A/N: Mewtwo is talking through telepathy.
"What brings you here?" Mewtwo asked. Just then, he started. "Ah, of course. You wish to battle me, to test out the strength of your companions?"
"Yes." Ash said, remembering that since Mewtwo was a Psychic type, he could read his mind. "You don't mind, do you?"
"Not at all." Mewtwo said, flexing his muscles, a gleam in his eyes. "It's been a while since I worked out."
Later, Ash and Mewtwo stood opposite each other in the middle of a clearing in the forest where Hoopa had brought Ash. "Greninja, I choose you!" the Kalos starter appeared in front of Mewtwo, his arms folded and his demeanor completely serious. Ash had briefed Greninja earlier about the opponent he would be facing, and if he was really the strongest Psychic type in the world…
"Let's start off! Water Shuriken!" Greninja leapt up and hurled pair after pair of sharp throwing stars of water at Mewtwo. The shurikens travelled at blinding speed towards Mewtwo, but he was ready for it. His eyes glowed blue as he used Psychic to implode the shurikens before they reached him. A cloud of steam formed between him and Greninja.
"Night Slash!" Greninja appeared through the smoke, a long sword of dark purple energy at the ready. Mewtwo's eyes widened as the super-effective attack struck him square in the chest, sending him skidding back a little. It hadn't done much damage, but Mewtwo had definitely felt pain from that attack. Smirking, it gathered a yellow sphere of energy in its arms, before firing it out in the form of a Thunderbolt. Greninja, who was in the middle of charging again, couldn't dodge and was hit squarely by the thunderbolt.
Greninja fell to the floor, downed by that supereffective move. As he struggled to get up, Mewtwo suddenly appeared in front of him; he swung his Night Slash blade, but Mewtwo easily grabbed the blade in his right hand, before his left hand struck Greninja squarely in the chest with a Dynamic Punch. Greninja cried out in pain as he was sent flying and landed in front of his Trainer.
"Greninja, are you alright?" Ash asked in concern. Greninja nodded, as he slowly struggled to his feet, his eyes burning with passion. "Alright let's do this. STRONGER TOGETHER!"
The familiar rush of power flooded both Ash and Greninja as the immensely huge whirlpool appeared around the ninja frog, before it split to form the large Water Shuriken on his back. Mewtwo stared on in amazement as he witnessed Ash-Greninja for the very first time, and the incredible surge of power he felt coming from his opponent was enough to get him fired up like never before.
"Let's go! Night Slash!" Greninja leapt at Mewtwo, so fast that it looked like a blur. Even Mewtwo himself was shocked at Greninja's speed; before he could react, Greninja had sent him skidding back, having hit him square in the chest with a pair of extremely long and wickedly sharp swords of purple energy. Mewtwo dropped to a knee, wincing from the super-effective damage that he had taken. Gritting his teeth, he fired off a massive Thunder towards Greninja. Ash's eyes widened as he saw the immense bolt of lightning headed straight towards him.
"Deflect with Sacred Sword!" Greninja replaced his Night Slash blades with a pair of even longer and sharper Katana Swords made of pure fighting-type energy, before placing it in front of him and with a tremendous effort, managed to deflect all of the electricity.
"Impressive…" Mewtwo said. His eyes glowed, before using Psychic to send a barrage of trees and rocks at Greninja. "Charge!" Ash bellowed. Greninja nodded and vanished, moving at such high speeds even Mewtwo had trouble keeping up with him. With Sacred Sword at the ready, Greninja hewed and slashed, reducing all the wood and rocks that came at him to splinters and bits before he found himself right next to Mewtwo. WHAM! Mewtwo's head felt as though it were going to burst with pain as Greninja slammed his katanas into the side of his skull, sending him crashing into a rock wall. He shook his head groggily, trying to clear his head, before glaring at Greninja.
"Your power really is something else…I never thought a fighting-type move would do so much damage to me…Well then it's time for me to go to an even further stage!" He held in his hand a Mega Stone, at which Ash and Greninja's eyes widened. Mewtwo closed his hand around the stone, and with a brilliant burst of light he had transformed into Mega Mewtwo X.
"In this form I am a Psychic/Fighting type." He told Ash, before beginning to fire Thunderbolts from his hands. Greninja ran and leapt about, and while he was still moving unbelievably fast, the rate at which Mewtwo hurled thunderbolts at him was making him harder and harder to dodge. Mewtwo formed a pair of yellow spheres of electricity before hurling a pair of full-power Thunderbolts at Greninja. The double Thunderbolt hit Greninja square in the chest, dealing copious amounts of super-effective damage.
"AH!" Ash yelled, placing his arms around his torso. It hurt, it hurt a lot. It felt as though his entire body were being fried alive. Greninja fell to his knees, weakened after that attack. Mewtwo swooped in, a Dynamic Punch at the ready. However, just as he reached Greninja, Greninja suddenly sidestepped and hurled the Water Shuriken on his back at Mewtwo, creating a large explosion of steam on contact.
"Now! Water Shuriken & Ice Shuriken!" The vortex of water surrounded Greninja again, and from the vortex came a pair of Water Shurikens, each as large as the one of his back, followed by a pair of Ice Shurikens, Water Shurikens that had been frozen by Greninja's Ice Beam. Mewtwo quickly recovered from that attack and used Psychic to shatter three of the shurikens, however he couldn't stop the last Ice Shuriken from hitting him, exploding into little sharp ice shards that dug into his flesh.
"Use Aerial Ace!" Greninja leapt at Mewtwo, his fists and legs glowing white. Mewtwo recovered and retaliated with Close Combat, the two Pokemon locked in a fierce brawl. Mewtwo tripped Greninja with his tail, before hitting him with a pair of rapid punches and a kick, however Greninja endured the attacks (they really hurt a lot) and uppercutted Mewtwo before ramming his foot down on his face with Aerial Ace.
"GRENINJA, DOUBLE TEAM!" Hundreds upon hundreds of copies of Ash-Greninja suddenly appeared all around Mewtwo, some with Aerial Ace at the ready, some with Night Slash, and some with Sacred Sword. They dashed at Mewtwo in a frenzied array; his eyes widening in shock, Mewtwo unleashed a full-force Psystrike that though destroyed many of the copies, left him vulnerable to continuous and powerful hits from Aerial Ace, Sacred Sword and Night Slash.
The real Greninja leapt back, watching to see if his opponent was down. The dust cleared to reveal Mewtwo covered in bruises and injuries, panting heavily.
"LET'S END THIS! ULTRA WATER SHURIKEN!" Greninja lifted the Water Shuriken off his back, and it spun of its axis as Greninja's watery aura surrounded it once more, converging into the Shuriken. The Water Shuriken grew to an incredibly, extremely massive size – even larger than the one Greninja had used against Alain in the Kalos League finals, and became a colossal, gargantuan, blazing-orange discus.
"FOCUS BLAST!" Gathering every last ounce of his power, Mewtwo created an enormous crackling ball of energy and hurled it at Greninja, as Greninja hurled the ultra-sized orange Water Shuriken discus. The Ultra Shuriken clove the Focus Blast in two like a fiery knife through butter and Mewtwo could only watch as it slammed down onto him, engulfing him in a titanic explosion of water and steam.
Ash covered his eyes from the massive energy bursts that were given off from that attack. When the steam cleared, Mewtwo was lying unconscious on the floor, having reverted back into his base form. They had won. THEY HAD WON.
Greninja reverted back to his base form, exhausted, as both he and Ash dropped to their knees and congratulated each other. "We did it, Greninja. We actually beat Mewtwo, and Mega Mewtwo at that." 'Kouga.'
Later, Mewtwo came around and congratulated Ash and Greninja. But that wasn't all.
"Ash…may I come with you?"
"WHAT?!" Ash exclaimed, not sure if he had heard correctly.
"The Pokemon here have all gotten strong, and no longer require my protection. I wish to go on an adventure of my own, and become even stronger. May I come with you?" Mewtwo asked seriously.
"Sure!" Ash said. Five minutes later, Mewtwo disappeared into an Ultra Ball that Hoopa had gotten through his rings.
"Welcome to the team Mewtwo."