The first step of dragon slaying is pretty established:
First you locate a place that the dragon frequently visits (i.e: considers it to be safe) and place an offering. This is a combination of various food items and one unlucky virgin (and usually serf) woman.
The theoretical reasoning for that is if the dragon's intelligent, he could probably use her as a slave. The virginity thing has no mystical layer to it, it's just signaling that "the product's brand new". I mean, what did you expect from a medieval society?
So, you hide until the dragon arrives to check out what's going on. Then you shoot at it with crossbows. The bolts are contaminated either with a poison or a disease.
And realizing that he's being attacked, the dragon charges at your team, kills everyone then collapses on the ground before losing consciousness.
The problem with poisoned crossbow bolts is that while the real damage they do is getting the poison into the bloodstream, a dragon is a DRAGON. It's huge (vent-snout length is 5 meters), and has a strong immune system. Poisons need time to act and you can't make a suicide squad out of mercenaries, plus suicide squads are outlawed (both in military and on the screen) here.
Size: 10 meters (5 meters of which is the tail)
Wingspan: 12-15 meters
Breath weapon: at around 5 liters of concentrated (0.2-0.1 pH) sulfuric acid; usually used as a fine-grain spray. The dragon's scales and most internal parts build acidic residue into their proteins, making them more resistant to the acid. Heartburn can still occur if this ability's overused.
Flight speed: 16.3 - 24.9 m/s gliding
Dragon's feel home just as much in water as in the sky, though they can't take off frequently, a dragon will usually be able to do an emergency escape after landing, since gliding doesn't take too much power thus allowing the creature to rest his muscles. Terrestrial capabilities are good, but nothing special.
Dragons have six limbs in total, two wings and the hind and fore legs, both of which have little value in combat. Though it's possible to amplify the power of the foreleg's strike by putting the rest of the body behind it.
There's also the most fearsome tail in existence, which can cause major damage (dented armor, heads blown right off) AND has a decent range.
The other major weapon are the jaws, coupled with a long neck, and the spurs on the dragon's wings.
As far as senses go, dragons have excellent smell and good eyesight, they can also sniff out poison in food, so there's that.
Note: You can't really cut a dragon's bone or scales. Crossbows have about enough power to penetrate deep enough to draw blood, but they can't cause serious damage.
Knowing that, just how would could a small group of people hunt down a dragon in medieval times without suffering heavy casualties?