Men of the people
Escort all hostages to safety
There are a total of eight hostages and they are all found in Chapter 9. You will know when you come to the hostages as there will be a short cut scene of them being surrounded by FDI troopers.
The first six hostages have to saved in the conventional way, but when you meet the last two, all you need to do is shoot some wooden support beams. The achievement will unlock shortly after when you unlock the door with your partner.
See the "Humanist" achievement for hostage rescuing strategies.