Mitsuo did not move to fight Isshiki. His body was simply too fragile for close combat against the full strength Isshiki. But his speed and lasers most certainly could keep up the 100's multiplier from all merged selves was able to do enough for him to not be speed blitzed and deal actual damage. But his durability didn't scale well as unlike those with supernatural energies defending themselves where more energy is more defense. All Mitsuo has is some very advanced metal for defence.
Instead Mitsuo went right towards Toshiko's head. The logic was simple, this domain was created by that head and destroying it should end the ability. From there he could repeatedly nuke the weakened Isshiki to death.
Five black rods intercepted Mitsuo's movements delaying him. And then from Toshiko's head the imbedded black rod activated. Toshiko's eyes rolled back as spinal fluid poured from his eyes once again.
"Neutral Limitless"
Mitsuo shot a massive laser right at Toshiko's head after those words, but the laser was blocked by an invisible barrier and completely unable to harm Toshiko. So he turned the laser towards Isshiki who had spread cubes all around to hide himself behind but he easily directed the laser to blast every cube in an instant.
As a row of explosions appeared Isshiki appeared next to Mitsuo with a fully winded punch, Mitsuo in that moment through out a jab towards Isshiki. But Isshiki struck his fist and obliterated his fist. Lasers shot out from Mitsuo's body all at once forcing Isshiki to retreat.
Mitsuo took a glance at his obliterated left hand and thought 'close combat is useless.'
Mitsuo began shooting lasers towards Isshiki who repeatedly blocked the lasers with his cubes.
'Why is that head still crying?'
Isshiki then summoned a cube the size of an entire mountain and pushed it toward Mitsuo to crush him, it was too large for the lasers to destroy instantly. So Mitsuo had to dodge to the side activating his thrusters.
Isshiki had caught up to Mitsuo again and threw a kick which Mitsuo blocked with a counter kick that folded Mituo's lower right leg with an audible crunch. Mitsuo realized that approaching Isshiki relying on his lasers as a deterrent weren't effective and began flying away from him at full speed. In response Isshiki began chasing, the gap between them wasn't closing or opening as their speeds equaled out.
Mitsuo began shooting out lasers and Isshiki shot out cubes and black rods like a tsunami fighting against a cone of pure light. Eventually after a solid minute of this exchange passed a laser bypassed Isshiki's defense and pierced a hole through his torso. However Mitsuo did not relent with his endless retreat within the vastness of the domain and kept on shooting lasers at a distance. He refused to risk another limb for what could be bait.
Isshiki felt frustrated as his bait had failed. This endless exchange was now a battle of endurance to see who could wear the other down with an endless barrage of attacks.
Mitsuo suddenly stopped shooting lasers towards Isshiki and changed his flight to dodge the barrage of projectiles from Isshiki. He started shooting lasers forwards instead with the same output he was back at Isshiki. Isshiki was puzzled, why did Mitsuo shoot forward instead of at him and risk getting hit?
Suddenly Isshiki felt his feet get burned away by lasers and then his legs. Isshiki screamed from the intense pain of his evaporated legs.
"How!" Isshiki stopped flying and redirected his cubes to defend himself from the lasers coming behind him.
Mitsuo smirked, "This space we're inside loops back around. But it's far too big for both of us to run laps around quickly. But my lasers can easily loop back around in 10 seconds. I noticed it just earlier when I was looking forward."
The domain expansion: Event Horizon was an massive looped space. It's spacetime took on the shape of a torus or what's better known as a donut. Isshiki was unaware of this fact because he had never truly forcibly activated Toshiko's domain ability before and studied it in extreme detail. And the limited range of the byakugan could never hope to perceive himself from such a distance where Isshiki would realize this fact. But it did not matter if Isshiki knew or not because only Mitsuo's laser attacks at lightspeed and his machine precision could take advantage of the looped space.
Mitsuo had found the path to victory. He began shooting out lasers up down and back. Isshiki was forced to move unpredictably and constantly send out projectiles to block the lasers coming at him from all directions. And Mitsuo kept on blasting away. Isshiki's body was being pierced and burned from the lasers that hit him, he would die any moment now.
But Isshiki smiled. "I told you I would kill you." The space around them began to contract and compress. "You forgot one important detail! That pathetic head isn't done yet!" The compression began to accelerate exponentially, stars were being thrown into a single point and the celestial objects around them stretched and warped towards that point. Mitsuo could see it, the size of the domain was shrinking, the distance of the loop was shrinking rapidly.
In the distance Toshiko's head that was still crying with the imbedded black rod spoke out the name of his domain's sure hit effect where no one could hear him
"System Collapse: Big Crunch."
Mitsuo stopped shooting lasers and stared wide open at Isshiki with eyes that could kill. In response Isshiki gave a polite smile. By now the domain was compressed to the extreme and each of them could see endless rows of themselves all around as if surrounded by mirrors in a room. There was only time for one action.
"If this is how it ends... Then I'm taking you with me!" Mitsuo took his right hand and pierced his chest, crushing the power core within himself. "In this tiny space let's see if you can handle an explosion of a THOUSAND STARS!"
Isshiki immediately shrunk to the smallest size he could and forced Toshiko to protect him from the domain's effect through the imbedded rod. However this was barely effective.
In the last moments as the domain's space collapsed Mitsuo's dying explosion was compressed and Isshiki and the beheaded Toshiko were forced to endure this implosion until the domain exploded from the outside in a brilliant burst of light like a supernova.
Mitsuo had died brilliantly like the stars he had admired. And his name would not just be remembered in the digital archives of Kagaku but in the legacy of science and the hearts of those he guided.