The School of Battle Magic
Something else to note is that if magic is a long-established thing in the world, then battle magic will be an established thing at some point in the history of the craft. There will almost certainly be witches that specialize in this kind of thing, as well as more scholarly types that sit back and study this field. Commanders and military strategists will consider their use in combat and their power as a strategic asset.
Tactics will develop based on the rules that are allowed to be bent in your magical practice as well as based on discoveries that have been made in the magical arts. We might be grounded in modern military theory and the treaties and conventions of our reality, but in your world those can and likely will be different.
Some things will remain the same, but others will be different just because of magic. Witches might be the equivalent of artillery, but they are not large and clunky equipment -- they are a living soldier and smaller and more fragile than a large cannon. This and more are things that will be considered when dealing with how to deploy a witch, and consequently how to protect them once deployed.
Unless this is the first battles where this is a thing, or you are just a random caster on a battlefield, Battle Witches will be trained to be on a battlefield and in a mass combat overall. They may not have as much to immediately worry about as they are not in the very front of the fracas, but they will have been taught the basics of their craft.
Even something as simple as martial arts, either unarmed or designed around their primary focus, can save a life and might even be incorporated into the discipline of Battle Magic. Using movement as the focus for gathering the appropriate magical energy likewise can prevent the caster from being a literal sitting duck.
This might also be how some train their apprentices in the craft of Battle Magic -- by being the core shield maidens of the primary witch, possibly in conjunction with more mundane soldiers/guards, they learn to rely on their body for defense and to not just rely on their magic.
Given time to evolve, even the spells are going to be different than their civilian counterparts. While the casting time may not be a factor that they can change, using motion to create the flows and shapes needed to enact the power. Even something as simple as capitalizing on the sacrifice of the witch's guards in her defense to help empower the spell faster could be integrated into this craft.
Not to say that being a guard to a witch is a one time duty with a 100% mortality rate, but that should a guard go down, a witch trained enough in the art could harness that fallen soldier's fading lifeforce into the spell to give it a power boost in some manner. The guard might be assigned so that should they all be killed and the witch live, the spell will have enough power to be launched regardless of casting time remaining. Whether doing so is advised is another matter entirely.
I would also expect the components of some battle magics to change over the course of research, trial and error, and just plain luck. All the dribbly candles and ominous chanting and octograms might be good show and good for civilian magic, but if all you need is a couple of sticks and an egg to do the same spell then that's going to be a lot more practical and usable in the chaos of combat.
As in this case, witches absorb the power through their skin, the answer by Kyyshak is a start of this idea.
It could be as simple as a witch not being able to direct absorbed mana fast enough at a scandalous level of (un)dress. The powerful and/or skilled might be about to work in nothing but woad made from the blood of a magical creature, but a battle witch on her first mission might be able to do her job in a one piece chest armour and a leather skirt. The exposed arms and legs represent the limit to the power they can channel at once and the armour functions both as a limiter to make sure there is no terrible side effects of overchannelling and as actual armour for defensive purposed.
This could inevitably lead to clothing (or a lack thereof) being a marker of capacity, as well as those wearing misleading amounts of clothing to disguise their true power. Another interesting possibility is the premise of tear-away clothes for an emergency power boost.
The answer by tinpact touches on the idea and it would be something that a military commander would almost certainly take into account when deploying the witchy artillery. This opens up protections in a different ways -- physical barriers and distance by hiding at the edge of a forest overlooking a supple run in darker clothes to hide themselves through camouflage as one way to protect a witch preparing an ambush.
If a witch was protecting a supply run, they might be hidden inside a moving cart, but mostly stationary themselves, holding a protective spell over it and the convoy, capitalizing on the idea that they aren't moving but their platform is. In this case, it is partially security through obscurity -- they will be hidden in the load and look just like another supply cart, as well as guarded by the typical guards that are guarding the caravan anyways.
Different mission profiles aside from being mystical artillery and casting the spells that make the peoples fall down will open up different avenues for protecting the witch themselves.