This achievement is not only the hardest achievement of 2017, it's one of the hardest to come out in recent years.
Mein Leben requires you to beat the game on none other than the Mein Leben difficulty, the highest in the game. Oh, and you must also do it in one sitting, in one life. There are zero checkpoints and zero continues.
Difficulty in Wolfenstein is no joke. Enemies aren't all that much smarter, but they will obliterate you in no time at all. Your health and armor will melt away in just a couple of shots, and running and gunning is almost impossible. Peak your head around the corner at the wrong time and you're dead, forget to kill one enemy and turn your back and you're dead. There is zero room for mistakes, and if you make but one it's all the way back to the beginning for you.
The worst part of this, other than it's insane difficulty, is that the hardest part of this game comes after hours and hours of playing. That damn courtroom level will make you scream in anger on even the most basic of difficulties, let alone the hardest one where a single mistake will cost you hours of work. It's recommended you practice the game countless times on the difficulty under Mein Leben just to be able to hold your own on this mode. It's going to take some serious dedication.
Somehow, someway there's a handful of dedicated Wolfenstein players out there who unlocked this achievement - about 10 of them. The unlock percentage for this is at 0.01%, and it's only that high because Xbox rounds anything under a hundredth up. In reality it should be somewhere around 0.0001%. Good lord.