Enhancement Magic is Easy
There are of course mages who can summon lightning to strike the battlefield, and to a greater extent, men with the ability to fling balls of flame from their fingertips. But for many, their greatest tool is the ability to cast small enhancements on their weapons and armor. A quick cast of Sharpness allows their sword to slice through un-enchanted wood and metal. A little extra power channeled into their boots can give a strong push off, allowing them to run faster and jump farther, darting across the battlefield. They are certainly better off than the average warrior. Unfortunately, even the most clever soldier can be caught unawares by a well placed rifle shot. If they can interpose their shield between themselves and the musket fire, the bullets will likely ricochet off, but the impact of a rain of musket fire will certainly force them back. And if a single shot makes it through, it could disrupt their concentration. Once they lose focus on their enhancements, they'll be peppered endlessly.
Lead Munitions
Magic is great, but even the toughest mage's shield is like paper to a lead ball. 523
They can't be fired far due to their weight, and the balls tend to be a little expensive, not to mention toxic if the holders aren't careful after using them, but there's no arguing with results. These rounds can't be used effectively for ranged combat against another rifle brigade, but can certainly help repel a group of magic-weapon wielding foes, provided you can keep your head down. Of course, this all depends on your being lucky enough to avoid being hit by a wave of thunder dashing you against the wall at your back.
Gun Hunters
Gunfire is all too common of a risk to mages. No matter how big your spell, a single round through the skull will put a stop to the casting. A few mages have taken exception to this, and instead seek to alter the course of a battle by removing their most deadly opponents. The best of these are not great archmages, but instead function as small units of guerilla tactic-employing warriors. Using magic to create a dampening aura, they stamp out the ignition of sparks with their presence. This means, of course, that they are unable to create any flames or gunfire themselves, but this doesn't matter to them. They're much more comfortable with eliminating groups of riflemen with a short-sword. A job made much too easy by the foolish reliance on working firearms, with only a silly little bayonet to protect them from these squads. 523