This is not from where the elevators are locked down, but rather from after the elevator sequence where you open the doors with your swords - simply get from the lobby after you get out of the elevator, through the studio, the offices, and into the boardroom. At the boardroom you need to simply get to the ledge outside for the achievement to pop. You do not have to kill everyone, just enough in the studio and outside the boardroom hallway to have the doors open up so that you can continue. I did this on easy on my first playthrough.
Do not reload checkpoint as I did taking the advice in Kruziik's solution when I was going to go over 5 minutes from the elevator lockdown - this will void the achievement and you will have to restart the chapter to have the opportunity to unlock it.
YouTube guide courtesy of GPAP360 - commentary is in Spanish, but it shows the location where the achievement conditions technically begin / end