***If you vote negative, please comment as to why. Constructive criticism is appreciated. ***
This achievement is supposed to unlock once you get all the other achievements in the game and therefore should pop once you unlock your 24th achievement. At least it does so for most players.
This did not unlock for me after getting all the others and even replaying the final case. So this is more of a warning than an actual guide.
At the time of writing, there are 45 gamers on this site that have unlocked 24 of 25 achievements, which should be impossible.
EDIT: I managed to finally unlock the achievement. The reason it was not popping after gaining all the other achievements has to do with the way the game tracks them through your game save. I found this solution after I contacted the Frogware support (which was very helpful, very nice developer btw).
It turned out, I unlocked the achievement for defusing the bomb during the Infamy case and quit the game after that. But there was no auto save at this point. When I resumed the game the next time, it started right before the bomb defusal – which I skipped, since I already had the achievement for it. But the new game save continued without ticking it off. So at the end, even though I actually had unlocked all the other 24 achievements, I had only 23 in the game save.
I am glad, I remembered that I had quit the game at this point and then skipped the defusal later. A quick replay of the beginning of the Infamy case fixed it and the achievement finally popped.
I hope this helps people in the same situation as me, although for you there might be a different achievement affected.