How would medieval army perceive the barbwires, artillery, propeller biplanes, rifles without actually been shot at? Or when been shot at? How would their commanders try to counter it without knowing the technologies?
Compare them to things they know. For example, you could call barbwire a "thin metal rope with sharp edges". Artillery could be "gigantic metal tubes with wheels that make loud thundering noises". Biplanes could be compared to metal dragons. Rifles could be "magic sticks made of metal and wood that spew fire and smoke". In regards to strategy, there's nothing they could possibly come up with until they've had several crushing defeats. Otherwise they just don't know what these weapons can do, so they'll be slaughtered. Only after they undertand that the big metal thingy does noise just before something blows up a few miles away, they'll be able to appreciate the importance of countering the cannon. Mobile light cavalry could surprise an artillery crew. Pistols would still allow them to defend themselves, but if the cavalry gets there quick enough, not many of them should die. Alternatively, cavalry with bows or crossbows could get close enough to release a barrage, then run away before the crews get their pistols ready. Barbwire will be the end of any non-armored medieval units. Plate, mail, and possibly even gambesson from lower leg to waist would negate the threat, even if they are still being severely slowed down. Gambesson would probably get stuck. Big heavy poles could be used to weight down the barbwire and hold it close to the ground, mildly negating it's effectiveness, but that requires preparation and time to do. If they're getting shot at while doing it, they'll become ground meat. Planes are a deal breaker. There's literally nothing they could come up with. Unless the pilot's an idiot and flies very low. In that case, a barrage of arrows and bolts could potentially harm the engine and make it crash. A balista would probably ruin it in one shot if they could somehow manage to hit it. As said, it depends on the pilot being stupid enough to fly low. Rifles are a serious threat. If the infantry has a somewhat capable leader, they'll learn real quick to have a few guys in reserve to shoot anyone who manages to get close enough to be a threat. The only solution for the medieval bois would be to somehow lure them into a close range scenario, kind of like the final battle in The Last Samurai. Of course, as with the pilot, it depends on how stupid and/or overconfident they are.
The simple fact is that they probably wouldn't counter them at all in an even battle. It'd depend on how many men are on each side and what the terrain and each force's positioning would be like. It seems to me like the best real-world instance to look at would be certain battles between native american tribes (from both North and South America) and Europeans. Horses were also used in warfare during both World Wars, so you could potentially look into that to try and feel out how horses might be used against heavy firearms. But like ElConesa said, no one on the middle ages would know what to do against modern weaponry until losing to it a few times. By then their loses may even be too great to continue fighting, anyways.
Its pretty much the premise of every guerrilla war, if you are lightly armed, and your opponent is heavily armed then you have to fight a war of attrition using traps and sting and run manouvres In forest or bush a bowman could hit a rifle platoon with one or two arrows and then be gone before they could return fire, bow traps, spiked impalers, punji pits etc - keep up the pressure, harras them, cut their supply lines, poison the wells etc. Lure them to land of your choosing and then hit them at close range, if you can get in amongst them a medieval force will be much more used to hand to hand with edged weapons
In terms of countering planes, ww1 aircraft were pretty basic, so they had to come down low to be much use in ground attack, so hails of arrows/bolts jets of greek fire, smoke to obscure visibility, wires strung across canyons or between tall trees. Raids on their airfields. Arrow fire as they were landing or taking off.
And don't forget to send in the prostitutes on intelligence-gathering missions. Mechanics are going to complain about the fuel having water in it, ordnance guys will bitch about the close call they had when somebody lit up a cigarette next to the powder shed, and officers will come in for a farewell fuck before they redeploy to the Shire the next day, moving the whole company down the highway through fucking Mirkwood, of all places, doesn't the general know that place is ripe for an ambush?
Applying everything said so far, I guess things would go somewhat as follows: 1. WWI bois appear and completely go to town with medieval bois. 2. Repeat 1 several times. 3. WWI bois rule the land now. Subtle info gathering starts. 4. Now the medieval bois know how those weapons work. Guerilla warfare ensues. 5. ??? 6. Profit.
I think there is a problem with the guerrilla warfare part. The British army had been fighting the Boers just 15 years before. Thus, many of the higher officers would have had some experience with guerrilla type warfare. (If this was 1914, then the regular British army troops would have been pretty good at countering it as well). In addition, WW1 soldiers could travel pretty light as well. At the start of the war, most were oriented towards a maneuver war and by the end of the war, this sort of mentality would come back in Operation Michael and the Hundred Days. I am not totally convinced that the medieval guys would have a clear advantage in hand to hand either. I think that once the range advantage of WW1 armies are established their generals will just put their troops in more advantageous situations as is only logical. The World War 1 generals would have had nearly 1000 years of accrued military experiences and theories. I know it is popular to criticize WW1 generals for sending out men into No Man's land and stuff, but it must be realized that often, they basically had very little choice under the pressures of political or strategic situations and they did the best they could with the shitty situations that the tactical realities presented.
"Sir Galahad, check out thine new shiny pokey rope." "Save that poor fellow who is trapped in the belly of that iron bird." "They have unleashed their thunder ballistas, hit thine deck!" "Runaway! They have thunder sticks!" As far as dealing with a more advanced army, when you have to rely on much less advanced means, you might want to learn Guerilla Warfare. Cause just doing an all out battle formation charge would get your ass handed to thee in great haste. Though they might fair alright, considering the Mil training of the time period in WW1, seeing as they were not conditioned to shoot people the way a modern army would. So it may be possible to just send a mass of men charging across the field. Sure they will take heavy casualties, but the infantry on the other side is less equipped to deal with a horde of guys with swords and pole arms at close range.
According to a couple sites I found with some quick research, cavalry was used successfully in the early stages of WWI, and Wikipedia says that:
Rifle (not being fired): "What is wrong with that sword?" Rifle (being fired): "They control the thunder!" Biplane: "The enemy rides dragons!" Barbed Wire: "They have lined their camp with a whip, covered in small swords." Artillery: "They have even more powerful thunder!"
Hmm... After decades of movies carrying this theme - I still love it! I offer the following considerations. Plenty of opportunity for comedy, tradgedy, viscera, even at the same time, with one as the leading theme. Modern armies stand overmuch on discipline and superior firepower, which loses the advantage of improvisation in the moment. Heroic as modern soldiers were, they typically had few years of training Medievel (?) knights, if that's what they are, were lifelong, gererations long, warriors, with devotion among ranks and person. They would read subtle cues from comrades, even at distance. Modern troops and armament stink of propellents, fuel, and modern hygene. They are constantly noisy, especially at night, and supply, support. Many midievel(?) had roman strategic perspective, and other lost to the world brilliance. They may respond quicker to the situation than expected. Ancient armies relied heavily on turning the enemy's weapons against them. It would be cool to see a small breakaway group (using tools they had even then, in pocket) break the wire and use each man in front to take a hit until the remainig string the wire around a large portion of the enemy, to make them shoot each other, etc etc. They would most certainly have scouts for recon, so would they blunder into a barrage? I couldn't figure if they dropped from the sky, like Ashe, or the exact beginning. Planes in the early twentieth were fairly (few?), unreliable, slow, and vulnerable. Where's the hangar? It has me wondering. I think the mdeivels are going to get their asses kicked initially, depending on wether or not you want it to happen, but oh, the fun romp you have brewing. Yippee!
"I say there old bean, is it raining?" (checks tank scope.) "No...just a lot of bowmen firing upon us." "Right-o, ahead full. The rain will stop shortly."
'Willow' 'Dune' They buried themselves then rear-flanked the emeny. "Well, my good fellow, they seem to have fled our tanks rather quickly. Do you hear one of our boys knocking? Probably time for tea. Be a chap and let him in!" (perfect for monty python or benny hill - or how 'bout 'V' ??
Just remembered, I read a book called Swords vs. Tanks a couple of years ago, but it was fantasy, so the magic gave the medievals an equalizer. I recall it as being a bit of fun, but I don't remember enough to give a good solid review or recommendation. Better than the cover art though.
Also, WW1 planes themselves were made of highly flammable materials (waxed canvas wings, was it?), so fire was a pilot's biggest fear. For the OP, read letters and journals written by WW1 soliders. The horrors of that war are exactly what you describe in your premise. "Modern" warfare clashed with methodologies that had been around since ancient times, and the mindfuck it caused for soldiers is heartbreaking to read about. That feeling would be easily translatable to the story you're writing.