The Weirdly Easy Solution:
In Jackson's version, Smaug has still been fucking shit up for hundreds of years, and he still has a weak spot on his belly, but the movies don't bother with Bilbo finding out about it. Instead, it's quite obvious, and Bard simply kills the dragon with a special dragon-killing arrow that the men had the entire time. In fact, it's mentioned that the weak spot was created by Bard's ancestor, who knocked off a scale using the special anti-dragon arrows.
Um ... so why has this dragon been built up as such a huge threat? The dragon arrow clearly works. There's lip service paid to how there's only one left in the area, but still, it does kill Smaug almost as soon as he attacks the town. Was there anyone watching this movie who saw a dragon with a famous weak spot attack a town containing an archer with anti-dragon arrows and thought, "Oh man! How is this going to end!?"
As an added bonus, the film also makes Bilbo and the dwarves' epic mission completely pointless. They don't discover the secret to taking down Smaug; they only piss him off and cause the town attack. And in that attack, the townsfolk kill him in like 20 minutes, which it seems they could have done at any point. Jesus, three movies for that?