To answer the dragon question in-character:
Boy, I've heard you and your kind time and again - 'If only the king hadn't been too weak to act,' 'The army should have been better organized,' 'if I'd been there I'd have taken up a longbow and helped bring it down.' You see the scarred scales in the king's halls, or the broken fangs, and think that it was some mighty dumb beast that was simply put down.
We didn't even know what we were facing at first - half a dozen towns, all isolated, ceased sending tax and news to the capitol. When we arrived, we found nothing but ash and the scent of brimstone - they'd been burned until even the castle stones had shattered. Trade caravans disappeared, and neighboring nations closed their borders, fearing what lurked in our lands. Whispers flew fast - some swore that the destruction could only be due to demons, while others claimed that only devils were so methodical, or that Efreet slavers were taking whole towns and burning them to hide their tracks.
Eventually, though, it missed a few survivors, and they brought the news: Dragon. Word spread quickly, and people panicked. Farmers abandoned their fields and fled behind city walls. Nobles hid themselves in great underground vaults, or else fled the nation. Food grew scarce, and trade failed entirely.
The king and his advisors took action - cartographers and astrologers surveyed the path of destruction, and found a pattern of sorts. While the targets seemed to share few similarities, when a wizard brought a record of the weather, it seemed the dragon was striking areas on the verge of drought, that they might burn faster and hotter. Even better, they seemed to locate its lair - a dead volcano in the southern mountains. The army was dispatched, every longbowman trained and readied, sent to areas the diviners found would soon be droughted.
The dragon didn't come - oh, of course there were skirmishes, sightings, even noble doomed defenses - but it never quite stuck where we thought it would. It would forgo the rich town for the poor and unguarded neighbor, or burn a valueless mill instead of the trading post we were guarding. And when we sought it out, we found that its lair was terrible to reach - landslides and earthquakes buried scouts, and all streams and rivers grew poisoned and sulfurous. We didn't surrender, though - our generals prepared for the terrible journey, willing to accept casualties if it meant forcing the wyrm out of its lair and towards our longbows
And then the wind shifted - blowing from the south, across mountain, forest and field, towards the capitol. That morning, the city woke to smoke - the yew forests, the rich farmland and even the rocky grasslands to its south were all burning. At first, we feared wildfire. And then came the roars.
Imagine it - smoke so thick and dense you couldn't see more than a hundred yards, and growing thicker as the wooden shacks of the poor district burned like kindling. Your captains, shouting contradictory orders, desperately trying to establish some logic to the events. Screams in the distance when the dragon would drop from the sky amidst any cluster of order, set the men to terror, the rush once more into the sky as it rained fire upon them. The fear spread with the fire, and soon there was no army protecting the city, just a crowd of terrified men and women firing uselessly into the sky, trying fruitlessly to slow the spread of flames, or simply hiding and weeping.
That is what a dragon attack is - not a beast descending clearly from a blue sky, but terror and flames cloaked in fog and smoke. That is why your army of longbowmen accomplished nothing against it. And that is why, when Ragna the Red and her mercenary band killed it in its lair, we forgave her her banditry and bestowed upon her knighthood - because she accomplished what we could not have done.
tl;dr if the dragon is fighting you on an open field where your army can fire on it, something has clearly gone wrong - it has no incentive to do that, and every incentive to be a terrifying hit-and-run raider.
