The guns were powerful. Unfortunately, they were small and couldn't cause too much damage if they didn't hit a critical part of the body.
It was especially important that they hit a critical part of the peak gem kings because the guns being able to output 100C only meant that they could injure the gem kings if they struck them. It didn't mean that the guns could kill them instantly with a single hit.
And as fate would have it, the gem kings only have one critical point in their bodies. This critical point is their heart core, and it is buried deep within their bodies.
As long as their heart core is intact, they will be able to survive even if their whole body is shattered. Because of this, any injury that doesn't affect their heart core cannot be lethal. This was why it was so difficult to kill the gem kings despite ambushing them with the guns.
