The battle had raged for hours, and the only way Alex had of hurting the Doctor was to hit him really hard. Fortunately, Doctor Demented wasn't making much progress hurting Alex either.
As Alex reconstituted his entire body for the umpteenth time, he considered his options. Was there anything that could permanently hurt him? It seemed like his heart was completely invulnerable, and he could regenerate the rest even more easily that Dr. Demented.
He saw the Doctor, silhouetted against the planet Neptune. He charged. accelerated himself to relativistic speeds. To the point where specks of dust shattered bones with their impact. And then, he hit Dr. Demented.
His bones didn't break. To say they evaporated wouldn't do the impact justice. Every atom in his body exploded in a shower of relativistic particles. It took about a second (in an inertial reference frame moving with Neptune) for him to reform.
He saw the Doctor had already reached a complete stop. But his old mentor wasn't making any moves. Alex decided to press his advantage. He forged a fleet of ships, hoping to hold the Doctor in place with their fire. Then, he could retreat, and ram his enemy at even greater speed. As he moved away, he saw, in deeply redshifted and Lorentz contracted form, his former mentor withstand a barrage of laser fire.
He decelerated, and prepared for another high-velocity impact. He rushed towards his enemy. Faster and faster. As fast as a proton in a particle accelerator. And then, in the nanosecond before impact, he saw the Doctor disappear.
He teleported away, Alex realized. Used another of the vast array of powers he had invented for himself. And then, Alex realized something else. He only has a few milliseconds before he slammed into a giant planet.
He slowed down as much as he could. It wasn't enough. When he hit the planet, every atom of his body was destroyed yet again. But his heart continued to plunge into the planet, setting off spontaneous fusion reactions as it went. It came to a stop thousands of kilometers beneath the planet's surface. It regrew a body. And the body prepared for battle.
Dr. Demented look at planet. Silly boy, fall into obvious trap.
Dr. Demented fall into more obvious trap. Hit in face by relativistic teenager.
"Not fair comparison," say aloud in vacuum of space. "Caught by surprise due to..." What is phrase? "Mind not focus. Not my fault."
Another part of Doctor take issue. "Entirely your fault. Your fault for having mental disease destroying brain."
"Is getting worse."
"Is way to stop it."
Mind is wandering again. Focus on matter at hand. Child is attacking me. Fight back. Sad. The child does not deserve to be repeatedly subjected to painful deaths. Is sad.
Dr. Demented observe planet. Watch as matter and energy move. Sees Crucible. Calculates trajectory. creates disturbance in energy field.
Alex noticed a strange sensation. Searing heat near his leg. As if he were being pinched, or like there was a constantly detonating nuclear bomb. What was it? He analyzed the sensations. Based on the two days he had spent studying science, the energy in his leg fit the spectrum of Hawking radiation from a micro black hole. It would be consuming a constant stream of matter, and converting it into energy.
Alex knew how to deal with such things. He willed his leg to disappear. The black hole, starved of material, evaporated in a fraction of a second.
Alex felt a surge of gravity waves. The waves vibrated his body to produce... sound. "Very good," the Doctor said. "Progressing in scientific studies."
"Thanks," Alex said, as he fired a laser at a piece of jetsam, causing it to explode and bombard his enemy with small impacts. "That means a lot."
And then, Alex had an idea. He rushed towards the Doctor, at a comparatively small fraction of light speed. The villain teleported away. Alex changed direction. The pursuit lasted for close to an hour (again ignoring relativistic effects). Eventually, the young god was able to grab hold of the villains armor. He jammed his hand in. Or, at least, he tried to. He grew, and engulfed his enemy. He pressed on every side, trying to find a tiny crack in the armor. Maybe in the joints or something.
Alex was sliced in half by a forcefield, the two halves were blown apart by a nuclear explosion, and the Doctor retreated several million miles through a wormhole, closing it when Alex was partway through.
Alex duplicated himself. The duplicates weren't as powerful, and couldn't regenerate, but they would last for a few hours. He created an army of replicates. A thousand demigods swarmed across the solar system. Doctor Demented observed them. They were interesting. When he decided he had watched them enough, he opened a portal to the center of the sun.
A jet of dense plasma shot forth, slicing the army apart. Another slice. The Doctor realized he was being inefficient. He opened a labyrinthine series of portals, and redirected the same beam through the entire army, destroying them all in seconds. All except one. Alex.
"You know," Alex said, speaking across the electromagnetic spectrum, "someone might conclude that neither of us can hurt the other."
"I would not conclude that," the Doctor said, preparing to teleport Alex into the core of a planet.
"Neither would I," Alex said, as he synthesized a small neutron star and threw it toward the Doctor.