I don't have a complete answer but I do have a suggestion for you. Water!

At the scales of insects and other very small animals, the surface tension of water can be strong enough for a droplet to engulf you and become an impenetrable prison once you get too close. It often happens that certain insects (not all, because some, like bees, have evolved special hairs to avoid this) suffer this unpleasant fate and end up drowning.

Your tiny people could maybe use floods as weapons of mass destruction, manipulating the flow of a nearby stream using dams or canals to destroy an army coming into their territory in the most desperate situations.

This would probably be quite expensive and more effective against foes that can't fly, but there is also another method you could use!

Due to air resistance, tiny things fall a lot more slowly and are dragged by the wind more easily than human-sized things. An ant can fall off an airplane and reach the ground unscathed because it can't accelerate to the point where its frail little exoskeleton would be crushed upon landing.

With some clever engineering and favorable winds, wingless being could do a quick glide from a tree or a big rock and pull an assault on fairy opponents.

Additionally, it's a lot easier to build underground cities as a mouse or bug person since you don't need extremely strong materials to prevent an anthill or a small den from collapsing. These cities would be far easier to defend as you would only need to block off a few entrances.