Middle-earth: Shadow of War

To get this achievement, you just have to unlock every skills. You don't need to unlock the upgrades to get it.

You won't be able to unlock Forged by War until Act 4.
Reason (spoiler free) :
Therefore, you shouldn't focus on this BUT ! Make sure you read this solution until the end.

THIS ACHIEVEMENT IS BUGGED.

On my first playthrough, my "Forged by War" progress was stuck at 30/34, even though I had unlocked all skills, all uprades, all collectables, all quests done and max game progress... 100% in absolutely everything except that I was missing a few legendary equipments.
So in case you weren't sure about it, this achievement is actually, truely and absolutely buggy.

So the first thing you should do is the following :
BACKUP YOUR SAVEFILE BEFORE YOU UNLOCK YOUR LAST SKILL (if you're already in 4th act) OR BEFORE YOU MAKE IT TO THE 4TH ACT.
The 4th act will begin after you complete a few storyline quests. Before that, at some point, the game will warn you that if you begin a specific mission, you won't be able to go back nor complete any other optional storyline missions afterwards. That's how you know you're about to start the 4th act.

How to backup your savefile (steam) :
1- Disable Steam Cloud saving. To do so, in your steam library : just right click on Shadow of War, click Properties, select the Updates tab then uncheck "Enable Steam Cloud synchronization for Middle Earth : Shadow of War".
2- Look for your userdata folder located in your steam installation folder (default is "C:\Program Files (x86)\Steam\userdata").
3- There might be multiple folders which names are a bunch of numbers. You have to find the one related to your steam account. If this is your own PC, chances are that the most heavily weighted one (or the one that has the most folders in it) is yours. Open it.
4- Inside it, open the folder "356190" then "remote".
5- You have to make a copy of the "xxxxxxx_ShadowOfWar.sav" file which is inside it. I just created a folder there and put a copy in it.
6- Likewise, you have to backup the same savefile located in "...\My documents\WB Games\Shadow of War". I don't know if the two files are strictly the same, but I don't want to take the risk corrupting the files, so I also copied it in a created folder.

Now you can unlock your last skill or make it to the 4th act.

If the achievement doesn't pop when it should, first make sure you have all 34 skills. If not, you'll have to use your backup file and try again.

How to use your backup file :
1- You might want to backup your actual savefile so you can continue your "main" playthrough later. Just follow again my guide instructions to backing up your savefile and make sure you put the savefile in another folder and that you keep track of it.
2- After backing it, as said, delete the two "xxxxxxx_ShadowOfWar.sav" files located in the steam "remote" folder and in the "My Documents" folder (the ones we made the backups from). You'll also have to delete the "xxxxxxx_#_ShadowOfWar.sav" (# from 1 to 5) next to one of them.
3- Put your backup files in there respective original folders.

This will give you another chance to unlock this buggy achievement.
My progress toward this achievement was stuck at 30/34, and it immediately unlocked when I unlocked my last skill on my second playthrough... yeah, I didn't backup my saves and I had to play the game all over again to get this last achievement.

Unlike me, by backing your savefile up you might save 30 hours playing the game from scratch again just because of a bug.

Don't forget to put your "main" savefiles back in place if you want to get your Talion back like where he was before attempting for the achievement again.

Please do leave a comment before disliking my solution. I just want to save your time and health, this game is quite long and is more enjoyable by playing after the end than replaying it from scratch !