Could magical defenses defeat conventional firepower?

Would firearms work against Death Eaters who could block bullets?

The counterargument depends on whether the wizard detects the attack and can handle several threats at once.

Magical defenses against bullets depend, in this scenario, on seeing or knowing about the attack. Artillery and the much larger non-magical population change the imagined balance, especially if the wizarding community remains divided. A military engagement would not require each opponent to wait for an individual duel. An expert facing twenty people at once is unlikely to defeat them all before one hits back. The same reasoning makes simultaneous threats more dangerous than a single opponent, even for someone with exceptional reactions.