In February 2024, after several tests, I finally discovered how to accomplish this feat, which had remained broken and unrepaired since a certain game update.
I posted the (basic) method on the various topics of the game forum where people were asking how to do it.

Recently, Squivels added a tutorial and a savegame to the method, with only 1 mission left to complete on the map in order to obtain the achievement.
Turning this into a guide will probably give more visibility to players who want to finish their game 100% this way.

Link of topic : https://steamproxy.net/app/518790/discussions/5/3815158655065977911

Basic explanation originally posted by NarTeKK on 29/01/2024

Today I was FINALLY able to complete 100% of this game !

This last achievement "Cuatro Colinas Arc" which has been broken for a long time on the latest versions of the game can be completed on an old deposit of the game:
518790 518791 5765192913839400327 ( 9 February 2021 - 09:59:56 )

I had to redo the cuatro colinas missions 3 times in total and on different versions before I found that this one worked =)

If you don't know how to proceed, here's a guide to playing with old deposits and therefore an earlier version of a game :
https://steamproxy.net/sharedfiles/filedetails/?id=889624474

Advanced explanation with save file posted by Squivels on 04/09/2024

I have a save file (https://drive.google.com/drive/u/0/folders/1dJqnIuw9Iop8Nh6OY3YSNfUtQvaoeB0e?ths=true) with one mission left to complete this achievement here if anyone wants to use it, you will of course need to get the older version of the game to make this work
(confirmed to work on a friend's account)

To that end, I've made a quick guide to performing this, it is far less detailed than NarTeKK's link so I still recommend reading that if you have the time.

WARNING! You can easily lose your game data or corrupt your game if you do this wrong, please be careful and take your time making sure to back up everything in 2 separate locations before attempting this.

Quick guide:

Installing older game version:
Go to your internet browser type in steam://nav/console to the address bar and hit enter
it should open up steam console, into that copy and paste:
download_depot 518790 518791 5765192913839400327
hit enter
it will take some time to complete and may seem like it's doing nothing (60GB) keep console open
when complete it will update and tell you where to find it
open your hunter cotw game install folder and remove everything
put the contents of newly downloaded depot_518791 into your now empty game folder

Replacing save file with mine:
Make sure cloud saves are disabled for cotw in steam
find your save file (normally C:\Documents\Avalanche Studios\COTW\Saves\
and move to a safe location (deleting this will lose your data)
download my save file (https://drive.google.com/drive/u/0/folders/1dJqnIuw9Iop8Nh6OY3YSNfUtQvaoeB0e?ths=true)
extract my save file
take my save file and put it where yours was (change the name to match the number of the one you removed)

Put steam in offline mode

Launch game, complete 1 mission, enjoy achievement.

Once you have the achievement reverse above steps i.e. delete my save and put yours back, same with game install data and lastly, put steam back in online mode.

hope this is helpful to some.
Again, big thanks to NarTeKK

Thanks also to Squivels for taking the time to complete all this.