Tukukkk

I have a group of pixies fighting a group of humans. The pixies are about 4-6 inch tall humanoids, which seems like it puts them at a big disadvantage unless they seriously outnumber the humans, or are using guerilla tactics, both of which I would prefer to avoid.

The pixies have a medieval level of technology, so no firearms. They are proportionally stronger than humans, and can lift at minimum their own body weight. They're as fast as a human, but more agile due to their smaller size. For the sake of this question, the pixies can not fly, nor do they have combat magic, though if magic would help them prepare a weapon beforehand, that is worth considering.

The humans have modern-day technology, primarily a couple of handguns between them for weapons. They are wearing regular street clothes, no special armor for this fight.

I'm unsure how many humans will be in the group when they get there, but I'd guess the ratio to be something between 1:1 and 2:1 pixies:humans. This is not a military v. military situation, just a small group of humans vs a small group of pixies.

Given that, what kind of weapons would the pixies use? What kind of advantage would those weapons give them?

If flying, firearms and magic are out, I'd say the best option for them would be poison. Perhaps they have access to poison dart frogs or else some plant based poison. Of course, having access to poison is one thing, delivering the poison is another question. They may use tiny bows and arrows, and the bravest of them may run to the humans and stab spears in ankles. (Or even climb the legs of the humans and stab them elsewhere). Without poison, such spear and arrow wounds would just be a nuisance, but with a potent enough poison, just a scratch may be enough.

The odds are still not in favour of the pixies though.

Short Answer: Anchoring Weapons and Bows

Regardless of what weapons Pixies use they need to be able to get their weapons to a height where they can attack human vital areas. This means the terrain will matter a lot. With the ratio given if pixies attack human's on flat ground they will be at a heavy disadvantage. If they fight in a forest where pixies can jump or swing from tress onto the shoulders of human's it will be much easier.

The other problem is that pixie weapons due to their smaller size will have lower inertia and thus even relatively useless armor for humans (cloth and leather) become fairly effective against pixies. You also need to consider the increase in the relative distance needed to get through a human's muscles and skin to do damage. Assuming a generally similar ration of sword length to height our pixies have sword a little under two and a half inches. This means that trying to stab a human in the heart is pretty much impossible for a pixie but cutting an artery in the neck is feasible.

The next problem is that human's size advantage makes unarmed attacks much more lethal from kicks to punches to throws. If you don't restrain a human it can use bare strength to rip through a group of pixies.

Combining all these problems and you're left with two options: using bows or other ranged weapons to attack the eyes and neck, using some kind of anchoring weapon to restrain and then scale humans.

Ideal Tactics

Half of the fairies aim to shoot arrows into vulnerable eyes in order to blind them. Another half will then shoot human's with tethered ballistae with one end weighed into the ground and the other end designed to be hard to pull out. These pixies will then scale the tethers in order to get onto a human in order to deal a killing bow by doing their best to open up the carotid artery.

Extra: You could try and trip humans up but personally I don't think this would work well unless you were using guerrilla tactics or catching humans off guard which relies too much on luck.

It doesn't matter that your pixies haven't invented guns because a bullet fired from the kind of gun a 6" critter can handle won't put down a 6' human. Or a 4'6" human for that matter. The pixies need something small that can cause big damage. Their best options then are poisons and incendiaries. A small amount of accelerant can start a big fire, not that it has to be big, even small burns are excruciatingly painful. Then of course you have different kinds of poisons and different ways to deliver them like blow darts, crossbows, catapults, poisoned blades. Better yet have them burn something that gives off poisonous fumes.