I did the Cse 2 skill, thoroughness and cunning 100%, but i actually didn't unlock this, it somehow glitched and didn't unlock, i did unlock the 100% achievement, but not this one. I think what you got to do, is just load your game date from the case you did this at the end and try not to get 100%. That might help, i will do this today, but this is just a solution if it's glitched ike me, and didn't unlock the first time.
Edit: This didn't actually work, if it somehow glitch try it again in next case. 90% shouldn't be too difficult.