doesn't break his arm or hand here (also knocks out accel)

Academy City’s #1 flapped his white wings and rose to an altitude of 5000 meters in an
instant.
Kamijou saw a pure-white light fly in a large arc and then start back toward the surface.
He had no way of knowing, but this top-speed rush using the white wings had saved many
people from the Star of Bethlehem at the end of World War III.
(My only chance is with a counter.)
Kamijou calmly moved his body.
(I can’t hit him with anything but my right first, but he can take a punch better than before
and I’ll probably be torn to mincemeat if I don’t finish it in a single hit.)
Before the clash, Kamijou crouched down and used his left hand to pick up a stone the
size of a baby’s head.
Accelerator’s reflection prevented any attacks not made with his right hand. Throwing the
rock would not even function as a distraction while that reflection was working.
But that was exactly why Kamijou did what he did.
In the instant of the clash between heaven and earth, Accelerator accurately attacked his
target. He saw every action Kamijou Touma took while holding his right fist toward the
heavens.
The monster did not even think about evading, but not because he was overly reliant on
his reflection. He was well aware how unique that right hand was and he decided to rush
in despite that hand which would pierce his absolute defense.
As long as he remained conscious and could continue his calculations, he would crush the
boy before him.
He understood the power of that hand.
(…?)
Or so he thought.
Kamijou Touma took an odd action.
He first made a cross counter, just as Accelerator had expected. Aiming for the center of
the chest rather than the face was certainly out of the ordinary, but it was within the range
of options he could take with that fist.
The problem was what came later.
Kamijou swung up the baby head rock with his left hand.
He first neutralized the reflection with his right hand and then he applied a tremendous
force to Accelerator’s heart, even if it meant crushing his own right fist in the process. The
impact from the rock was much greater than anything he could have caused with a mere
fist.
(Ah.)
Accelerator’s mind went blank.
With a tremendous noise, the thoughts that controlled his calculations cut out for an
instant.
***
The battle had ended, but the pain had not.
Kamijou Touma groaned as he walked across the greenery of the glass plain and trudged
through the deep white snow.
“Gh…”
Two things tormented him.
The first was his shoulder. The intense force of the #1 falling from the heavens and onto
his raised fist had almost dislocated the shoulder.
The second was his wrist. Smashing the large rock into his own hand had put a heavy
burden on that fist.

- New Testament Volume 10 Chapter 9 Part 5,7