Hi,
depending on the road you take in this DLC, this will be the 2nd or 3rd group of hostages you free. This achievement belongs to normal story progression and is unmissable
These hostages are located at the north part of the map. Basically what you have to do is surviving a large group of enemies in combat. After you have beaten all the goons, place some explosive gel on the water tank in the middle to free the officers.
The achievement will unlock right away.
If you need additional help, please go to minute 20 in the video below
(video credit: Batman Arkham Videos)
Hope this helps you guys! :-)
Any improvements? Please let me know in the comments below!