Firearms fire projectiles at an exceptionally high speed.

Assuming a standard distance of fifty feet away from an opponent, firing a gun at them will cause the bullet to literally hit the opponent before they have time to process the reaction. Humanoids all have the fundamental flaw of having an absolute minimum reaction time based on their neurotransmitter-based biology, and guns exploit that incredibly well. The only thing one can do to avoid immediate death by gunshot is prepare a defense in advance.

I'd also like to point out:

Magic and Technology are [b]not opposites[/b]. They are two methods of achieving the same goal - one of which is reliant on mana (or another energy that takes the place of mana) and another which is realiant on using non-mana sources. They are both simply methods of achieving an effect.