Middle-earth: Shadow of Mordor - Game of the Year Edition

There are 10 Acharn Legend missions required for this achievement, all of which are available in the primary Story mode. They are very straightforward, stealth-based missions.

You can also do these whenever you'd like for the most part. A few require certain upgrades to be unlocked, but what I meant was that you can do these before you play the story or after you beat the game. I don't think it upgrades your dagger in any way other than aesthetically, so it does not matter when you finish them all.

However, be aware that they give a lot of XP per mission, which could unlock useful upgrades that do make the game easier.

Also, to make these considerably easier you should go into them with a few Upgrades + Runes. For some of these you are allowed to use the bow, so I used the following Upgrades, which made the challenges not all that challenging:

Attributes
Shadow Strike/Lethal Shadow Strike: Allows you to basically teleport around a map. Without a doubt the most useful upgrades in the game.

Quick Draw: Combined with Shadow Strike, you can teleport around and get instant kills even while in combat.

Runes For The Bow
Ascendant (Epic) - Reduces focus depletion by 50% (comes with the GOTY version).
Deadly Archer (Epic) - Increases headshot damage by 25% (comes with the GOTY version).
The following runes can be any level, but higher levels are better:
Keen Eyed - Recover focus for stealth kills with the bow
Broken Mind - Chance to recover Elf-Shot with Drains or Brands
Sylvan Arrow - Chance to recover Focus for a headshot kill

All of those combined will basically make you an unstoppable, undetectable ghost. All of the challenges are doable without becoming unstoppable, but I played all of these Legends after I beat the game and had all of these upgrades, and breezed through them with minimal effort.

Here are the challenges you will need to complete. Credit for this list goes to Zasta 360GameTV for thinking to transcribe all of the information before completing the achievement unlike me.