Firearm with rubber rounds (if he’s hit in the right place)?
In “A Connecticut Yankee in King Arthur’s Court”, Mark Twain had his hero use a lariat and a trained horse to humiliate a knight. He also uses the electricity gag (and I use the word “gag” advisedly – electric fences and knights don’t mix.)
The problem with using chemicals such as pepper spray or CS gas is that it’s a pretty understandable technology, even if the knights don’t know exactly what it is. It would be regarded as mere trickery, and they’d kick your ass for using it.
Depending on how much infrastructure and time to prepare your hero has, I'd suggest a big ass electromagnet. Bury it underground, get your timing right, and hilarity ensues.
Failing that, a concussion grenade would be no fun for someone wearing a metal helmet. A riot gun with the rubber rounds Badtz Maru mentioned, or the expanding beanbags would possibly be effective if well-aimed.
If you really have to improvise, I'm thinking that spraying fast-setting epoxy at a knight in armor would be kind of amusing. Think rusted "Tin Woodman".How about a flare gun? It would bounce off the armor, yet scare the poop out of anyone who didn’t know better.
I cast… FIREBALL!
Eh, the knight is a trained warrior. I think trying to pull such a thing on an armored knight is bound to land you in the hospital (and in those times it might have meant a leach or two would get to feast on your blood!)
The magnet might work, but you’d need a source of energy to power it.
If you managed to rust his suit of armor (if that can be done), he might have trouble raising his visor after he chopped you to pieces. Otherwise it would hamper him none, except that he’s have to come up with a fortune to fix his suit.
I like the rubber bullet idea. Would be REALLY hard ot hit a vulnerable area though, as even the arm pits and most joints are covered in mail.
Well, I don’t want to totally rip off Twain, or the thousands of others who have farmed this territory. But you have given me a number of different tacks to consider.
The initial draft of the scene had our hero approaching the knight, nailing him in the chest with a stun-gun, and stepping back in triumph. The point was to show that he could drop the most powerful of opponents with just a touch. Now I will have to rewrite it to use something smaller, and not obviously a weapon. Pepper spray seems to be the best bet for the next draft. Or maybe police riot “tangle-foot.” Hmmmmmmmmm…
A powerful electromagnet may be cool. Have the Hero go up to the knight, flick on the magnet and stick it to his chest. All the armour would stick together and the knight wouldn’t be able to move. Hero could then just shove the paralyzed knight over and restore him with a touch (flicking off the magnet and removing it).
A small napalm flame thrower would be interesting. Hero could spray it at the knights helmet and leave him blinded and freaking out because his head is slowly boiling inside a metal cage. Although from what you’ve said I figure your looking for something more subtle.
You could also make up a chemical that crumples and cracks the plate mail around the knight. That would be intimidating.
Well yeah, trained to cut or flail someone to peices with a gigantic knife or jab some kind of pointed stick/spear through you. I’m not so sure they trained as hard for the double-leg take-down defense or being thrown (not that they would need to).
And if he’s unarmed and just standing there hands-on-hips laughing behind his armour, I think being picked up and dropped on his head would be rather humiliating.
Then again such tactics sure aren’t high-tech or new… but they could be new to the eyes of people use to seeing people get hacked up like X-mas turkey.
Bah, just have the good guy light a little fire cracker and slip it through the knight’s eye slit in the helmet magician-style: thunder touch!!!
Remember that in a one-on-one “challenge” type battle, there are rules of knightly honor to observe. And they prohibit any use of a weapon that leaves your hand.
So bow-and-arrow, crossbow, trebuchet (stone throwers), etc. are out. (Those were weapons of the lowly foot soldiers, not knights.) And this would seem to apply to any kind of modern propellent weapon, guns, rubber bullets, beanbags, acoustic weapons, etc.
Even a pepper spray is questionable under the rules. It would be rather like having your friendly alchemist make up a pail of acid for you to throw on your opponent. Certainly not likely to impress those watching with anything except your own dishonorableness.
Your original idea of a stun gun would probably be OK, since the wires still connect back to your hand. (The studded ball at the end of a chain was an allowed weapon, since it never left the knights hand.) But then the wires would have to be visible, which might undercut the impact of your scene.
Remember that in a one-on-one “challenge” type battle, there are rules of knightly honor to observe. And they prohibit any use of a weapon that leaves your hand.
So bow-and-arrow, crossbow, trebuchet (stone throwers), etc. are out. (Those were weapons of the lowly foot soldiers, not knights.) And this would seem to apply to any kind of modern propellent weapon, guns, rubber bullets, beanbags, acoustic weapons, etc.
Even a pepper spray is questionable under the rules. It would be rather like having your friendly alchemist make up a pail of acid for you to throw on your opponent. Certainly not likely to impress those watching with anything except your own dishonorableness.
Your original idea of a stun gun would probably be OK, since the wires still connect back to your hand. (The studded ball at the end of a chain was an allowed weapon, since it never left the knights hand.) But then the wires would have to be visible, which might undercut the impact of your scene.
It’s called a sword 
A polearm?
Unarmed combat was a part of knightly training, yes. As was armed combat against unarmed opponents. If he has his sword drawn, the fool who attempts to close in without a weapon is going to be sliced in two.
Unlike in today’s society, brutality, war, and combat was an everyday thing. These people were warriors. They would not be impressed by WWF wrestling moves. Centuries of kill or be killed hand to hand combat makes them capable of very nasty, very efficient martial arts.