This achievement can be unlocked pretty early in the game but is missable.
Right after the scene with Zsasz and the guard you will talk to another guard who tells you about some trapped people in a room filled with gas. As soon as you made your way around to the room (through a vent shaft again) you' ll need to save 3 people in this room. 2 guards and 1 goon.
The first guard you need to save is the guard who is hanging from the same platform you are standing on when you entered the room.
The second guard is hanging from a platform directly above you. You need to press
to get yourself up there.
The third person would be the goon. From the position of the second guard you need to glide over to the position of the goon which is a lillte bit lower than the position of the second guard. As soon as you have saved all 3 people the achievement will unlock.
If one of these guys dies you need to dashboard and start another game to get Arkham shut-down and hope for an earlier checkpoint to kick in.
If you need further help, please refer to the video below.
(video credit: RajmanHDGaming)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!