Given enough time to prepare, even a small, low-tech group of barbarians can wipe out a heavily armed army of trained soldiers. Surprise is a massively useful weapon; no matter how well armed or how well trained a soldier is, if he is running around screaming and wetting himself, he might as well be a rabbit.
So, you want to make this highly trained army into an army of rabbits. Fear and confusion are your best allies, and the best weapon to cause fear and confusion is fire! Fire is loud, hot, bright, and unpredictable. It's easy to find, and hard to fight. However, you also want plenty of theatrics. The more unexplainable events there are, the harder it will be for reason to overcome uncertainty, and the easier it will be to scare the life out of an army. I know you just wanted melee weapons, but sticking with a motif makes it much easier to plan in the end.
First, costumes. You need three different costumes: Fire, Smoke, and Ash.
- "Fire" will be the hardest to design. You'll want thick hides, pitch, and your biggest warriors to wear them. Paint the warrior's face and arms with dark mud; not only does it look scary, it also protects from flames. Paint the hide with pitch. Just before battle, light the pitch on fire; the warrior doesn't need to have the hide on long; first impressions are key. A towering warrior wreathed in flames is extra freaky. Pitchforks and tails would be a huge bonus.
- "Smoke" is easy. Those men should have the same dark mud, but no burning cloak. Instead, they should wear smoke bombs - smoldering rags or lit bottles of saltpeter and sugar. Lots of smoke, no fire.
- "Ash" is the easiest yet; their faces, armor, and anything else exposed should be painted bone white. Chalk dust is perfect for this, as is real ash.
Second is weapons.
- Smoke and Fire costumes should have torches and darkened melee weapons, capable of doing a lot of damage in tight quarters. Weapons like axes, short swords, and maces would be the best choice.
- Ash costumes should have short swords, knives, or other very-close-quarter weapons.
Third, prepare the ambush. Find an area you know the enemy will be marching through. You'll need about four hours to prepare. If you skip the trapdoor holes, you could probably get ready in as little as an hour if you have the materials available. Dig shallow holes in a field, glade, valley, or flat area, enough for approximately half your group; each hole should have a wooden cover and enough dirt and grass to hide it. Place one man in each hole and cover it up. Then, spread a mixture of dry grass, dry leaves, and fuel made from animal fat over the field, careful not to get any near the trap doors. Added bonuses would be "rocks" made from balls of pitch and flour, wax, or even pinecones - anything that burns hot, bright, and/or with lots of smoke.
Fourth... wait. When the army approaches, with just a few well-placed touches from hidden torches, the field becomes a wall of fire. The dry grass burns quickly, lighting the animal fats and pitch; it will also create clouds of smoke. Once the enemy is distracted by the flames, attack with the Fire and Smoke warriors, pushing the enemy back into the flames. Finally, once the flames begin to die down, the Ash warriors under the trapdoors can leap out and join the fight.
Once the fight is over, burn any evidence, including bodies; leave nothing but ash.
"Bored from our long march, we were glad of the soft grass beneath our feet. Suddenly, someone at the edge of the field began shouting - fire! Faster than a thought, the dry grass at our feet burst into flame! At first, we tried to stomp it out, but it spread faster and hotter than we could bear, and the field was soon a burning inferno! I had barely scrambled to the edge of the field when I saw the first of the demons, huge creatures with blackened faces, wreathed in flames, swinging burning brands, and screaming in an unearthly voice! I dove to the ground and lay as one dead as the demons cut through my comrades. The demons drove my countrymen back into the fire, laughing at the screams of the dying. For a brief moment, I found hope as our soldiers pushed the hellspawn back, but suddenly the very earth beneath our feet broke open, and warriors of smoke and ash leaped forth! I ran, somehow escaping in the chaos, but I was the only one; the rest were dragged down to hell. When I returned, hoping to find survivors, there was nothing left but ash and bones..."