In my world I have two sides battling each other. Both sides consists of Soldiers, however, one side tends to have a smaller number of far more elite solders while the other side has mages which only participate in important battles because it takes so long to refill expended power, but are very devastating when involved in a fight. Mage's mainly are good at making things go boom, not much else.
Solders in this world can Channel, enhancing their physical traits by forcing power through themselves. They can enhance their reflexes and stamina greatly, as well as smaller but still significant boosts to speed and strength; they also tend to be able to survive smaller wounds while still fighting as long as they Channel. Channeling is tiring, and how long one can keep it up depends allot on how much they channel (one can burn themselves out to be super-powerful for a minute, or fight an hour or two with only a bit of a boost over regular skill etc). The point is solders are operating on a degree of superhuman power, how much varies greatly from person to person. This power mostly is seen in enhanced reflexes helping them to see, anticipate, and counter attacks rather then superhuman feats of strength.
Mages can not channel, making them much weaker physically. Enemy mages tend to be held back away from the frontline, they are rare and powerful enough that they aren't placed in positions where there is a reasonable chance of being hurt or killed. The 'good guys' consist of the most elite squad of which have two mages with then, the older is a bit of a unique case, a competent fighter who uses his mana primarily as a sort of anti-mage by protecting against, and defeating, enemy mages. The younger 'apprentice' of them I want to eventually get hold of crossbows which she starts to use to help out in a fight when she can't cast spells (it takes time to build up mana, and so she may have to go some or all of a fight without casting any spell when low on mana).
The archer/mage does not need to be overly lethal with her crossbow's at first, She does it only because she can't stand doing nothing during a fight but knows it would be stupid to risk herself in a melee battle. However, I would like her to offer some sort of utility using her crossbow even when still mastering them, and once she develops to be a good archer she would be less dangerous then her fellow solders with them, but she should still be useful. She must have some proven success in killing, or at least hurting, enemies to make it worth her continuing to practice and develop the skill.
What I'm looking for is a justification for making arrows/crossbows limited enough as weapons that they are either never used by the enemy army, or are of limited effectiveness against my elite squad. I also want to prevent the good guys being able to use arrows to kill dangerous enemy mages from afar when it's too hard to fight to them. This squads is pretty much the anti-mage team responsible for stopping enemy mages, but raining arrows until one hits a mage is too easy to make this squad interesting. Thus I both want to limit them while keeping them useful in some cases.
The mage/archer may be fighting at a shorter range, firing her bows at a front line that is much closer then an archer would usually like to be, so a solution that makes arrows viable when very close but limited at range would be great.
I originally wanted to use some sort of "anti arrow" barrier spell as justification for this difference, but the way mage spells ended up working, it doesn't seem like they should be able to create a spell that stops arrows without it draining their magic rapidly.