My fairies are hunted for food by basically anything that’ll eat bugs, mice, and small birds. Humans also target them because wild fairies are seen as a nuisance (and a tasty nuisance at that). How can they fight off all of these creatures? Magic is very limited, and their magic is too weak to make a difference, anyway. Their tech-level is very primitive, and their intelligence is close to that of a human’s, however they have weak memory. They’re also rather weak, however strong for their size, and have the endurance if a hummingbird. They can fly, but not for very long, but they can fly pretty high. One strain, which lives in a forest, is 4 inches tall on average, and another that lives in a dessert is 2 1/2 inches tall on average.
Bees and wasps are also delicious. If you try to eat some, the rest will go after you. Bees and wasps are pretty good at what they do but fairies are better because they are smarter. Bees and wasps know what eyes look like. Fairies know the eyes are still there after they close. Bees and wasps have stingers several millimeters long. Fairies have needle-like barbed spears several centimeters long. Bees and wasps have venom that hurt. Fairies have vegetable toxins that will paralyze you after a few minutes. Bees and wasps are satisfied to run you off. Fairies will watch from a distance while you stagger blindly around. Then when you stop moving they come down and finish you off, and eat the parts they like, and not necessarily in that order.
- Poisoned nets and snares to kill fast-moving hunting birds and rats.
- Poisoned knives that kill ambushing spiders, insects, frogs, and lizards.
- Poisoned arrows that irritate, daunt, and drive off larger hunters: Foxes and cats.
- Poisoned elixirs and bit of night-burglary to make human foes weak, bedridden, and irrelevant.
Many common plants worldwide contain caustics or poisons in varying concentrations. The fairies may not be too smart, but they know how to find and harvest poisonous plants, how to concentrate and store the poisons, and how to safely apply and use them
Build your fairy society high in the trees, out of reach of surface predators, and below the reach of birds of prey. It seems there's no reason for a fairy to leave the fortifications. So they won't.
By choosing trees which provide fruit up high, and are difficult for humans to get to, the fairy society can thrive forgotten.
The desert fairies can fortify rocky outcrops, or burrow below ground for shelter and a water source, growing mushrooms for food. Come to think of it the Forrest ones can also burrow too.
I get the feeling this isn't the answer you're looking for - you want the fairies to fight back.
They can't realistically pick up guns or bow and arrows or anything (assuming a fairy can delivery 1N of force, you'd need like 100 to pick up, load, and fire a small crossbow). But they have numbers, they can move in the forest, and, because they're fortified and the attackers come to them, they can dictate the terms of battle, eg set traps. They should also withdraw away from direct confrontation, and try to trick the enemy into coming into their traps.
A trap will be set, villager walks near it, if they're going to walk past it, a fairy appears to attract their attention, and flys away. Human chases fairy right into trap. Fairy's feast on the corpse of humans!
The villagers fighting your forest fairys will be akin to the US fighting the Viet Cong. The "walking through the forest setting off traps killing you one by one" part anyway, (not "Tet offensive"). Spike pits. Sharpened sticks with poison on them. Trip wires spraying a cloud of poison gas. That sort of thing.
Worst case - take the predators down with you
Each fairy straps a small satchel of poison in a biodegradable container to her inner thigh with a skin coloured peice of cloth (or animal skin) when she goes out. If captured, it will be hard to spot the poison and remove it, and, if eaten, she at least will kill the beast that ate her.
I chose skin coloured cause I assume humans wont want to eat cloth, so will strip fairys before cooking and eating them - we eat fairys nude? This is weird to think about, and very rule 34.
Because fairies are intelligent I will assume they follow a K-type reproduction cycle despite being small; so, they can not afford to throw away countless lives in meaningless battles the way ants or bees could. It takes a lot of time to raise and teach an intelligent child no matter how big or small you are so I will assume they can not afford to rely on pure advantage by numbers tactics.
Also, your fairies need poison, I know this has been brought up before, but in nature, it is the one advantage that allows a creature that small to kill something much larger than itself. If your fairies don't have poison and they don't have r-type reproduction, then their only hope of survival as a species is to be very good at avoiding conflict which seems to not be the point of this question. With poison factored in they can stand up against larger species on more-or-less equal ground.
Terrestrial Predators: Since fairies can't fly for long, things like cats, snakes, etc. would have plenty of opportunity to ambush them while they are on the ground working or resting. Much like humans did, I suspect the fairies would preemptively hunt these species to prevent being hunted later. These predators would in the grand scheme of things not be that big of a deal to fairies because they could just fly over head and snipe them with poisoned arrow or javelins. It would not take long for these predators to learn to actively avoid fairy territory the same way wolves and bears normally avoid humans.
Birds of Prey: These are much more dangerous to hunt because the fairies can not automatically fight them from any position of safety. That said, having humming bird like flight is a huge asset. A fairy can no more outrun a hawk any more than a human can outrun a charging bull, but what they can do is out maneuver them. When fighting birds of prey, fairies would adopt a fighting style like mid-air maddador combat. Holding their ground to get the bird to commit to an attack, then side-stepping at the last moment with a thrusting attack into the predator's side. This would be a hard skill to master so the fairies would probably have a special knightly order or something similar who devote himself to the art of protecting the village from birds, but with the right training, such warriors would rarely lose a fight.
Humans: In a one on one fight, this could go either way. Humans can armor themselves pretty easily with something that can block the tiny weapons fairies use, which would mean fairies have to work hard to find an opening. This means getting close enough for a human to catch and crush, but their agility also means... it's doable. That said, on a battlefield, the fairies would have a huge advantage thanks to a battlefield mechanic gamers often call "doom stacking". This is where one side can focus all of their strength on a very small portion of the enemy's forces. If an army of 1000 humans gathers against 1000 fairies, the fairies could concentrate all of their forces on 10 or 20 humans at a time. Some fairies would draw the human's attention and just focusing on not dying while the other fairies sneak in for the kill. By forming a super tight battle group, the fairies can overwhelm just a small portion of the human army at a time until the whole battle is won. Lack of endurance will play against them here, but they could swap out as the battle progresses. A few hundred fighting at a time while the rest hide among the bodies of the dead regaining their strength.
Bats: These are by far the worst threat to fairy kind. They come at night in huge swarms and have the agility and reflexes to hunt animals like fairies very effectively. Your fairies can't dodge a bat, can't outrun them, can't outlast them in a war of attrition. When the sun goes down, fairies hide from bats, because there is nothing else they really can do. Since I've already established that humans would be at a huge disadvantage against fairies in a war; people would probably domesticate bats just for the purpose of killing fairies.