If your pixies have the same flight performance as a pigeon or a duck, then it'll be impossible to hit them in flight with bullets, but it will be easy to take them down with bird shot. So, the enemies would simply start using hunting shotguns with bird shot 12 gauge rounds, then probably switch to full-auto shotguns like the AA12.
Bird shot has very low penetration power because the projectiles are tiny and light-weight. So it is ineffective against humans, and even worse on orcs because of their thick hide, but it is extremely effective against small light creatures.
If the pixie stays still, then at short range the shotgun is even more effective, but at long range a 22ga varmint gun would work very well too, which means the enemies would be able to carry a ton of ammo.
So, before thinking about offensive tactics, the pixies would have to think about staying alive long enough, and use their main strengths, which are flying and small size, to their advantage.
For example a bunch of pixies carrying an AK47 would be clumped together and slowed down by the weight, an ideal target for birdshot. This would fail before firing a shot.
Since they fly, they should attack from high above, either at night, or during a sunny day. It is easy to aim a shotgun at, say a 20° angle to shoot a duck, but quite difficult to aim upwards (vertically) especially with the sun in your face. When aiming up, you can't rest the shotgun on something, and you'll probably trip, fall on something, and during the fall the shot may go off in the face of someone nearby who is on your side.
Then they would need some form of lightweight armor. It does not take much kevlar to stop bird shot, especially if they fly 50m above the field, the projectiles will be slowed down by aero drag and gravity. And placing the "shield" below the pixie would protect the most fragile part (the wings) better, and it would be more practical, since a flying creature can't carry something heavy in front of them: it would cause them to rotate until the load is on the bottom and the wings are on top again. Likewise if one pixie carried a gun, even a small one, it would always point down due to its weight.
Now if the pixie evolved to fly they would fly with their body in a horizontal position like all birds do, because their wings generate lift towards their back, not towards their heads. To fly while standing up, you'd need hummingbird-style wings, that can generate lift in the proper direction, but then they have to flap back and forth, so you can't have arms, they would get in the way.
So it makes sense that the natural flight position of the pixie is horizontal, so for them it would be natural and comfortable to look directly down, and attack in this direction, while for the non flying enemies it is very uncomfortable to look and attack directly up.
They have to fly high enough to be out of range of flamethrowers. But the use of flamethrowers against pixies would not be such a good idea anyway, as it is not advisable to fire burning napalm in a vertical direction : it comes down eventually.
Therefore the ideal pixie weapon would be some form of bombardment. You could make a mini frag bomb with a fuze in the front that detonates it when it hits something. Add some fins in the back so it points down when dropped. There is no need to spend weight on propellant, since gravity will do the job. All the weight should go into high explosive and frag.
Keeping the shield/armor idea, you can have several pixies carry an armor plate, IF they're all hidden behind it. So the plate would be horizontal, and the pixies on top. It can also serve as a basket to put the mini bombs in, but the bombs should be designed to not explode if the thing is shot.
Ironically, this would be deadly against tanks and bunkers, because the guys inside will be unable to fire upwards, and they have bad visibility, so the pixies could just land on top and throw bombs through the openings. Same for trenches and foxholes.
Since the enemies would cover their trenches with something like fabric, that can soften the landing of the mini bombs enough to prevent them from detonating, they also need a timed fuze like a hand grenade.
Now, if you find the idea of pixie aerial bombardment not "fantasy" enough, you can replace the bombs with something more "forest"-oriented, for example a hive full of hornets, or some form of fire ants. The pixies could even raise such insects for the purpose of using them as weapons, so through selective breeding they could get insects that are friendly towards pixies, or with venom that doesn't harm pixies, but the insects would be quite deadly against other species.
Just think about it, you sit in your bunker and suddenly a hornet hive is dropped from outside through the tiny window, that would make a mess.
Also, see youtube for what happens when someone shoots a hornet's nest with a shotgun.