You know you can just Coup de grace the Linnorm.
I actually didn't know about that, every time i tried to use Coup de grace on something the game wouldn't let me so i kinda stopped trying it early in the game and never realized what it was all about. I'll try to see if that works.
It's DnD, it doesn't scale to your plebeian level.
As i wrote i haven't played tabletop games so it might feel fine on such a setting. However FWIW i do not remember anything like that in the DnD-based computer games i have played - the closest would be enemies being immune to some types of weapons in NWN (which also happens in Pathfinder: Kingmaker) but i can accept that you may not be able to damage some ethereal being with a regular weapon or some dragon with thick scales with a simple dagger.
What i was seeing in the game however wasn't that, it was my party causing a lot of damage on a dragon that had fallen and while its regeneration couldn't keep up with the damage, it still wouldn't die.
There is a sleeping bandit camp to teach you the coup ee grace mechanic in Act 1
This doesn't ring a bell so i don't think i encountered that.
and you had regenerating trolls the whole Act 2.
The branded trolls can be killed in a variety of ways though, this is why - as i wrote above - i tried several approaches, potions, bombs, etc on the dragon. I might have missed something since i only tried it once.
In fact it is exactly because the game teaches you via the trolls that there are various ways to kill them that i kept trying to kill the dragon.
Don't blame your shortsightedness on the game.
How do you even read what i wrote as shortsightedness? I explicitly wrote that i encountered the dragon accidentally. Even then what annoyed me was the
mechanic of immortality that can only be beaten with a specific thingamajig. I don't care where that comes from or even why it is there, what i care about is that i had an unexpected and otherwise enjoyable fight of the sort that i rarely see in RPGs that was soured at the end because of something i felt like an arbitrary and nonsensical requirement.
That said if the dragon can be killed with Coup de grace after falling (which i don't think i tried) then i take everything i wrote about this topic back and it was simply me ending up forgetting a mechanic that was introduced early. In fact i am curious to try it now.
Also i wonder if that is the case, if branded trolls can also be killed like that.