You've done your diligence.
You've read the old forums, the reviews, the warning signs.
You've watched enough walkthrough videos, that simply points out what the flavour text had already explained.
You know it can be done. Because others have said so, as it was written to be resolved.

BUT you're here now, hoping for an answer. That will lead to unlocking this achievement.

1. Unlocked during Story Mode.
   Majority of posts & sources point out that it was unlocked during Story Mode, first runthrough.
   
2. Jumping Off North of the Bridge (Away from the Player)
   
3. Jumping Off South of the Bridge (Towards the Player)
   
4. Reinstalling STEAM + GAME on another PC.
   Small reports of this working.
   
5. Unlocked during Co-op gameplay.
   Less than a handful of sources, during story mode.
   
6. Disabling all EXTRAS prior to entry into the MT. Doom.
   A number of posts advises that having Extras may affect the RNG gods.
   
7. Using EXTRA Codes to resolve other achievements MAY affect your chances.
   Referring to Dead Marshes Treasure Item Count affecting one (or two) achievements.

1. MOST (if not ALL) Story Achievements can be Unlocked during Free Play.
   
2. EXTRAS change the way the game behave.

Of all the forums I've eventually started to gloss over while researching - there was this singular informative post regarding collision. MY MISSING PIECE of this puzzle.

Originally posted by jatowar:

I had the same problem and just got it. I noticed watching a Youtube video about the achievement that when it unlocks, your character is breaking into multiple Lego pieces. That wasn't happening to me: mine was just "falling" and it even looked like it was hitting like an invisible wall when jumping (not going futher to the background).

It looks like a collision problem to me. So I modified all the graphics properties: back to 1024x768 4:3, V-sync on (I had it off before), low textures. It worked after that. Maybe it has been just luck, but it may worth a try :)

With this piece of information, I was able to unlock this achievement with the following game actions.

NOTE: I had successfully entered one of the five codes to resolve the Dead Marshes Treasure Items count prior to this attempt.

1. Changed my Graphic Properties (1024x768 4:3 V-Sync On) accordingly.
   
2. Prologue in Free Play Mode
   
3. Most of the session with Non-Relevant Characters.
   
4. All Stud Specific EXTRAS turned on throughout the whole Prologue.
   Because I needed to unlock the 10b Studs Achievement.
   
5. Prior to Entering Mt. Doom, through the gates. I had Isildur as my active character.
   
6. Moved farthest away from where I ENTERED the scene but not activating the last cinematic.
   
7. Ensured I didn't have the SAFE FALL EXTRA toggled ON.
   
8. JUMPED off South of the bridge. (Towards the player)
   
9. Achievement GETS.

I believe this will help MOST future (and current) players seeking to unlock this achievement.
GOOD LUCK & CONGRATULATIONS