In this story, the world is in peace, until the Lightning nation attacks. Because of the peaceful period before, this particular nation under the attack of the Lightning nation is able to learn about the creation of the lightning. The general idea of electricity is sufficiently known:
- It can be grounded.
- It can be redirected using metal.
However, due to lack of other magic/bending techniques, this nation is left to their knowledge of metallurgy and electricity (and other knowledge) to handle the invading Lightning nation.
In this nation, they can produce small electricity using a waterwheel, through a mechanism they currently don't know - electromagnetism - (it just works!) to research electricity. There is no electrical device other than basic wires, so the research in this area is boring and particularly undeveloped. This can be reproduced easily, but they have no idea why this work.
This nation is particularly famous for the melee soldier: ranging from hand-to-hand martial artist, rogue-like dagger wielders, to knight in armor, and archery and other projectile-based is not developed very well, so they must depend on their melee soldiers.
The Lightning nation, confident in their lightning bending over soldiers without magic/bending ability, only sends lightning benders to invade them (with catapult and battering ram for sieging castles).
Lightning bending works by creating a low resistance channel of ionized air toward the targeted location (only air), then release the accumulated, all using fingers - Avatar-like. The power is only comparable to real lightning in 20 meter.
An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 500 megajoules of energy.
All lightning bender soldier able to do this, with more experienced users capable of extending that range. The power of the bolt diminished exponentially with a maximal average range of 50 m. Using the ability is at-will and you can discharge from your 4 limbs, less than 1 second needed to release a bolt, and recharge cooldown varies from 3 seconds to 10 seconds for greater bolt.
How the attacked nation can prepare their melee soldier to defend against the lightning strikes?
I'm not asking about how can they win the war, or kill the enemy - might do that in their sleep by sending assassins.
Encasing the soldiers in Faraday cage obviously out of the question. I don't think carrying a metal rod for grounding is capable of protecting the user against a lightning, though I'm free to be proven wrong of this.
Please consider the medieval setting, their limited knowledge of electricity (no electrical device yet, only basic wires), and unavailability of magic/bending at their country. The answer also should be practical for a melee soldier. Cookie points if you can redirect this back to the enemy army.
This means that even inside a Faraday cage, if you are close enough to its walls, the arc flash will still injure you.
The math is done there if you are interested.