Actually when you think on it ALL dragons greatest vulnerability comes from inside. That is to get beyond the hardened armour and magic they have there is really only one good way. Have it eat someone with 6 pounds of c14 and a detonator on him. Its suicide but a lot cheaper than the tons of sammies and mages that would die trying conventional methods. I suppose poison might work or if you are creative coming up with a batch of the rare dragon flu and DMSO might work as well.
Actually, thinking on it , dragons second vulnerabillity would be inactivity. Meaning that they rarely do anything personally, they hire others to do it for them. By eliminating those closest to the dragon you close in its circle of trust and remove its abilities to influence others greatly. Of course you still need to stay alive.