Addai67 wrote...
(husband)
That seems like a reasonable explanation for why the plot is the way it is. However if someone wanted to be nit picky they would say the area he attacked was on its back. not its wing. If it crippled its stamina why were its hit point bar unaffected? (It starts with full hit points).
I'd urge you to watch that cinematic again.

When the archdemon tries to knock Riordan off by slamming its back against a tower, Riordan throws himself off its back and drives his sword into its wing in an attempt to stay on. His sword then drags a gaping hole all the way through one major membrane (that hole is still visible in the final battle). Riordan falls to his death at that point, and you can see the dragon falling in the background at the same time.
So Riordan not only gets an attack in on its back, he also disables a wing. This is why you see the archdemon only being able to leap small distances on Fort Drakan, where it crash-lands.
Since other arch demons have been killed in battle, (even though it might take a years and years to eventually off them). I might suspect that like like other dragons and giant monsters that maybe they just like to mix it up once in a while. Like any warrior chief they may simply find hiding behind their army too dull to stand....
With the exception of the first, the reason the other Blights take so long may well be because it's so difficult to fight an archdemon (flying, taint-flame spewing dragon) on equal terms. Riordan said at the gates that for them to have a change of killing the archdemon they'd *have* to get to high ground and attempt to draw its attention, which sounds like the creatures aren't in the habit of just landing in any old place where they can be conveniently attacked.