Before you cross the bridge at the end of the dungeon, make a save! It's important because you will probably have to restart it a few times.

The achievement is problematic, because in this section of the game, a certain script may or may not be activated.
If it hasn't been activated, the camera will move up, showing you the boulders falling into the pit, but your character will keep walking as if nothing happened.
That's what happens most of the time

In this case, restart the save until you see the following:

If the script has been activated, your character will fall while nearing the end of the bridge.
Now it works correctly

In this case, do not restart the save. Instead, walk up the wooden stairs and activate the trap when the first troll gets close to the small rat.
If you've done everything right, you'll get the achievement instantly.