Ok, I read some answers and I think there are a few more methods I can add or expand.
"Sisters of battle"
Who says witches has to stay alone? Lets say, in battle, all the witches forms covens. 3 witch group. One for long range defence, blocking arrows, redirecting explosives and poison etc...
Other is for close range assault. Butchering or frying or doing some magical bad thing to whoever comes close.
Final one, is the guarded one. She is guarded from pretty much any sudden attacks. She can do whatever she wants/circumstance needs.
"But "Where" is the witch really?"
Lets say some witches on the relatively back side, creates illusions to mess up enemy troops and effectively cloaks the front-liner witch, giving her chance to reposition, run, or hide among the chaos. Maybe every witch channels some part of a one giant illusion spell that messes up every soldiers minds and those areas get stronger near the witches so... no one even wants to try to kill a witch.
Maybe the witch was in the area that siege weapons blasted a few minutes ago. You would think that area is empty, but invisible witch
with talent for fire, is standing there.
You can expand this idea as, a mist only your people can see through.
"Skimpy cloth is strong, alive and angry."
Imagine her cape, her long scarf,or whatever skimpy but long clothing or dress she wears is actually a living creature that can block arrows, hard to kill, and can chop off heads, cut off hands, quick to act and just feeds on witch's magic power. Since, witch literally wears nothing but the creature, she can gather enough mana to feed the creature for the day before battle, and continue casting.
This flesh was burrowed anyways...
Maybe the women you see as witches, are souls of witches possessing unfortunate, or willing (up to you, the writer) women and there are more than enough backups at the back of the warzone. So, killing one of these girls is a waste of time due to killing a literal mage is really hard, and resource consuming while they can just... come back in a few minutes. So you will now see long distance magic battles where witches themselves are fighting among each other and warriors are fighting among each other.
Wrong weapon of choice, buddy.
Maybe, to really kill a witch, you must be a witch. Or something similar. And since you can't truly kill a witch, they just come back and attack again. Incapacitationg one would, create its own challanges. So, you need to cut open a path towards the witch and let your witch finish the job. Hard work really.
Now it s a magic duel, and magic duels rarely require full metal armors.
So war tactics hugely revolves around witches now.
And some witches can hide among the soldiers, drop the heavy armor in the blink of an eye, use her "partial" immortality to dish out some truly heavy damage.
Let witches ride armored horses. Heck... armored wolves. Let them be mobile, hard to hit, easy repositioning, and gives them a good chance to carried to safety if everything else fails.
Maybe an armored horseman is carrying the witch with the same horse.
Magic + swordsmanship, mobile, armored.
Witches are basically like a siege engine. So lets treat them as one. Build a freakin siege tower, or some other armored moving thing witches can stand on before the battle. And put the witch on them, carry them with the help of these armored platforms with wheels. Let the witches cast, and be protected from attacks with the equipment.
But I don't want to. I would never hurt this witch.
Charms, seduction, or similar mind spells messing with soldiers, causing them to simply not want to hurt witches. Mabe lowering their fighting spirit, or creating fear, paranoia, insanity that making them turn against each other.