this can be devastating to formation especially to cavalry.
so to counter this thing i believe they will use literal meat shield like <PERSON> era, or like strapping light animal inside of the shield or put them as meat armor, at least light enough to be able to lift easily for a person and not aggressive, since OP say life Horse also get affected, (i reconsider to put them covered inside the shield to make sure the chance for it to still "alive" to counter the magic, definitely will be strapped shield type and quite conical or curved to fit the animal in it or a pavise kind of shield, if want to put them inside the shield, if the person cant withstand tickles or something wriggling on their main body) considering the anti magic shield is expensive, and i believe it is better quality or not as awkward compare to strapping animal inside it, probably only affordable or best suited for the commander or general and the front line of cavalry in wedge formation without losing momentum or crash with their front line (depend on the size of the magic shield or the protective range of the anti magic, since i just found out horse get affected, but cavalry is more likely become obsolete otherwise, since horse is scaredy cat, and i am not sure strapping small animals as armor can work or worth it).
or it will be guerrilla warfare or like modern warfare considering everyone can use this magic.
i agree with <PERSON>, shield at least will be given to all the unit to deceive their opponent.
regarding fortress siege or defense it depend on the strategy or tactic, since both can use it. and the wall is good enough to hide you from getting targeted.
after seeing the edit that it include firearm or explosive weapon, then i believe guerilla warfare or modern warfare or at least trench coat warfare is more likely to developed, even <PERSON> kind of formation will be devastated in this case, and armor definitely obsolete (excluding modern armor if OP has it), since firearms will dominated more, though it still depend on OP anti magic shield and normal shield, but i am skeptical napoleon type of formation is worth it in this case. 358