Some Civilians are tricky to harvest. I won't specify each ones location, only the total per level and the ones who pose a problem to harvest. Each harvest is worth 500 points in the statistic screen. For those who have problems with these description of the civilians you can find another guide for this here.
Mission 1 Research Lab:
Once you freed your brothers and sisters you enter some hallways with two floors. You'll notice a civilian on the top level in a closed room. This one CAN'T be harvested and doesn't count towards the achievement.
Mission 2 Colony, 3 civilians:
After climbing up the ventilation shaft and exiting the sewers, you will encounter one Marine with motion tracker and a civilian who escapes to the next room. If you force the door open he will shoot himself. Proceed through the hallway and enter the vent above the obstacles and slowly enter the room.
Mission 3 Refinery, 8 civilians:
After the first room with Marines, you pry open a door. One civilian will run away and trigger the elevator with two marines in it. There is also another who runs through the second corridor and sometimes kills himself if you are not fast enough.
Mission 4 Jungle, 4 civilians:
After you've been ambushed in a closed off room you have to open the gateway to the Hive. On the top level next to the switch is a civilian. If you go near him on ground level he will turn around and use a grenade before you reach him. So after you enter the room, cling to the wall or ceiling, ambush him, and quickly press to harvest him.
Mission 5, 7 civilians:
A quick overview to where they are:
- The first is straight ahead and one level down next to an android.
- The second is on top of the middle pyramid next to an android, but the civilian might kill himself by falling off the the edge.
- The third is next to a pressure plate guarded by an android. Kill the android by suprise and DON'T walk on the pressure plate or you might burn the civilian.
- The fourth is on the opposite site of the first one but alone.
- The fifth is walking around starting behind the wall of the fourth.
- The sixth and seventh are inside the closed off pyramid.