100 men, 100 meters - if the demons are even a little smart, this will be very one sided. Many of the solutions are to use typical combat tactics of the age with lots of arrows, pikemen, and cavalry. I also see lots of comments and answers that twice as fast is not that much. Both of thes are wrong...take a look.
The fastest man alive today runs the 100m at about 28 mph. Now double that - 56mph! Assume the demons don't train for the olympics and have only 60% of that speed and they are still travelling at over 33mph or 50fps. Think about that for a second - oops your dead because a demon 50 feet away just slit your throat. They will have covered 100 meters in 5-6 seconds.
The highest high jump is a bit over 8 ft and the longest running long jump is just shy of 30ft and neither athlete was travelling over 33mph! You could assume easily, that these figures would rise by a minimum of 50% at that speed and strength. This means that traditional pike men will be all but useless as the demons simply hurdle them.
Let's further assume the demons are small in stature, maybe 5'4", and with their natural armor giving them a little bit of weight, you could estimate a creature in 120lb-130lb range; let's say 125lb on average. According to these calculations, the demon could generate more than 1500lbs of impact force after only 6 seconds (assuming 3ft of stopping distance)!
So the demons have closed the 100m distance between 5 - 6 seconds, are hitting like a freight train and your arrows are doing nothing because the demons are naturally armored in chain mail. Now you are in close quarters battle and hoping the arc of your sword will be enough. The problem is, you've barely got half a second to react and move. At 33mp, the demon has moved another 16 feet in the half a second you decided to do something about it. But you are heavily armored and the reaction time has been doubled. You swing your sword, but too late, the demon has already passed you by more than 30 feet and killed 3 of your buddies who never saw him coming.
So how do we beat them?
We're going to need some preparation for this. The field needs to be prepared to defeat the demons before they get to us.
- Prepare it with pitch in bands 5 meters apart, within 20 meters of the humans.
- Create some punji pits for them to fall in and stay in.
- Spike the field, pounding 100s of spikes to make running difficult
Once that's done, we need to even the odds a little with some big cats with big teeth, big claws and fast reflexes.
We need arrows, but they need to be pitched so they burn. The demon's mail will minimize punture wounds, but we may set a few on fire. These will also light the pitch bands we set in the field the night before.
We need different armor. It needs to be light, it needs to stop knives, it needs to be layered leather. The necks need to be high and vitals all need to be covered - torso, armpits, wrists, groin, thighs.
Everyone gets a bow; even if they only get one arrow in the air. Some will get 3-5 in the air. At worst this will be a distraction and the field preparation is that much more effective. At best we take out 10 - 20 % before they reach us.
Everyone gets a short sword and small shield. Swords are all sharp and poisoned. Hand to hand is going to suck, but hopefully lightening the armor and armaments will give us enough reaction time and speed to counter and overcome. The poison may not win us the day, but might kill after we're gone, giving others a chance.
Finally, everyone gets a buddy. Close quarters combat needs to be back to back, literally watching each others backs.
All this may still not be enough, but I think it gives us our best option with the limited resources and lack of geographical advantage.