Hoosier427

Poison would work if you knew what type of poison to use. However what's poisonous to you and me could be very different to a dragon. For example dark chocolate can be poisonous to dogs and certain plants are poisonous to cats that aren't poisonous to dogs or people. Without capturing a dragon and testing (good luck with that) it's hard to figure out what would work and what wouldn't.

So what are some other ways a dragon could die? Could you drown a dragon? They are so strong and powerful I don't know what you could use to keep under the water unless it was severely injured. Could lightening striking a dragon kill it? Again not enough info and that would take an act of God which u can't rely on.  I think fire is your best bet considering I saw one post say dragons killed smaller dragons with fire and if I remember correctly, it was said wildfire would burn anything, wood metal flesh it didn't matter. 

If it were me I would pick a spot and basically try to lay a trap for the dragon. Try and use the natural terrain to my advantage in whatever way I could. Then you have to figure a way to get the wildfire on the dragon itself, the more the better. If your up high maybe u get lucky and and your able to pour it on the dragon as it flies up the mountain. Or u cover the ground in it and try to lure the dragon to that spot. Set it ablaze with a fire arrow and pray like hell it works. I'm sure a lot of people would die in the attempt if it was even possible, however that's the approach I would take.