he's pretty delicate past his shield

Kamijou was not especially good at fighting.
In a fight against delinquents, he could win with 1 on 1, he was in danger with 1 on 2,
and he would unhesitatingly run away with 1 on 3. That was the extent of his skill.

But even so, Accelerator could not reach him.
The punches Kamijou was throwing did not have his weight behind them. They were
diversionary punches that put more strength into pulling back than in the punch itself.
In boxing, they would be called jabs.
But even so, Accelerator was being hit strongly.
Accelerator had never once lost which also meant that he had never once been in a
proper fight. Because his power was the strongest, he had never had a chance to use his
athletic abilities normally. Kamijou could not utterly crush even a delinquent in a fight,
but he could easily beat the hell out of a sheltered boy who had never been in a fight.

- Toaru Majutsu no Index Volume 3, Chapter 4 Part 6