This was because there was a tremendous noise and Terra of the Left’s body suddenly exploded.

No, technically Kamijou did not see the instant Terra was blown away.

An orange flash came down through the ceiling right above Terra. The pillar of light was about 3 meters across and, the second it hit the floor, a terrible blast roared through that room of the Palace of the Popes. Kamijou’s feet were torn from the ground in an instant and he flew several meters back like a puff of dust. Itsuwa and the powered suit were caught up by the blast too and flew back with him.

[...]

The area Terra had been lying in had become a swirl of lava. A few meters of the stone floor had been turned to a glowing orange pond and what looked like the same stuff was dripping from the huge hole in the ceiling

[...]

The supersonic bombers that had attacked Terra at the Palace of the Popes had been powerful, but to Terra it had all been one kind of attack and therefore easy to defend against with his “precedence”. It was only multiple attacks at once that he feared.

Toaru Majutsu no Index Vol.14 ch.4 pt.8