(Warning this guide/video contains spoilers!...)
The video below is mine, and this is the steps to get this achievement.
Difficulty doesn't matter, and it always works the exact same way.
What i describe here is exactly what i do in the video:
Run by the door in the beginning, you'll get the attention of one of her elite guards. Subdue/kill the guard,get the disguise and dump him. Then find the keycard, and follow her to the panic room...
IF you actually missed the timing i did, just head right inside the room and wait for her, might take a few min but she'll eventually go there.
Then you actually get to destroy something beautiful... (The Achievement doesnt pop for me because i already had it.)