Do they need to react though? Surely there's some sort of kinetic barrier spell they could use. Just checked and there's protego, a shield charm that blocks other spells and physical objects. A high mobility special forces wizard squad would have one person responsible for casting protego while all the others use avada kedavra on everything that moves. They could also use some sort of explosive spell to destroy cover, walls, obstacles and fortifications. All while shrugging off bullets, shrapnel and pretty much anything muggles could throw at them short of a nuclear missile.