There are a total of three prisoners. For this achievement, at least one prisoner must live and one prisoner must die. The fate of the third prisoner is left up to you.
Prisoner #1 - I'd recommend killing the first prisoner, solely for the time it saves you. Allowing him to live takes a bit more time, as you have to stack boxes awkwardly up to the ceiling for your escape.
Prisoner #2 - This is the easiest prisoner to save, as it takes next to no effort. Simply put Slide #4 in the top slot on the prisoner's cell, and put Slide #3 in the bottom slot. Afterwards, pull the lever and it will let you advance on while saving the prisoner.
Prisoner #3 - Ignore the third prisoner and allow him to die. During the chase/escape sequence with the third monster in the water area, not taking the time to save this guy will net you extra time to escape without dying.