Aliens vs. Predator

This achievement wants you to harvest all of the civilians possible. From what I have been told thus far, this achievement does and doesn't work like collectables. Either way, I have tallied up the amount of civilians in each level that can be harvested. I also made notes as to the people who will give you a bit of trouble, if not handled properly.

RESEARCH LAB: 0 Civilians
Yeah, there's that one person in the room as you and your siblings escape but this person is inaccessible.

COLONY: 3 Civilians
2 of the civilians are easy to get. One is a bit trickier. He will be with a marine and mention how he would rather kill himself than deal with one of "those things." If you go down the vent and force your way into the room he's in (by pressing X at the door), he will shoot himself with a pistol. Instead, kill the marine in the hallway. Climb over the debris and look for a vent to crawl through. Follow it until you're above the room the civilian is in and climb out the vent, then snag your harvest.

REFINARY: 8 Civilians
The first 4 civilians are easy to get. The fifth and sixth are a bit tricky since its all about the order you attack them. After getting the fourth harvest, you must pry open a door. Doing so startles the next civilian. DON'T HARVEST HIM! Keep running past him into the next room and harvest the guy working on the vehicle. Doing this will cause the man you ran past to call for 2 marines. The marines are easy enough to deal with. Afterward, harvest the remaining guy and carry on with the mission.
In my experiences, harvesting the guy right after prying the door open made the guy working on the vehicle run off and disappear.

JUNGLE: 4 Civilians
Most of these are easy to get. One will join up with marines and follow them around, but once his guardian is gone he's easy to get. The tricky one in this level is the man in the security room (second floor of the big building). He's guarded by a marine with a smartgun. That's not the problem, though.
Getting too close to the civilian will make him arm a grenade and kill himself. For this guy, climb on the ceiling and press LT to focus. When the green circle is on him, press RB and quickly press X while he's on the ground. You should get the harvest without much trouble.

RUINS: 7 civilians
6 out of the 7 are easy to get. The tough one, for me anyway, was the civilian examining the vent at the top of the middle temple. He's guarded by an android and the android doesn't like to follow your hissing. Killing the android will cause the civilian to run off a ledge and kill himself, thus negating your chances at this achievement.
What I did was slowly crawl up the temple, behind the android and I pounced him (LT+RB), then I spammed X on the civilian. During the harvesting, I was getting shot at but not too much. After the animation was over, I was nearly dead. I quickly sprinted to a safe area and hid until my health came back.
Instead of getting him through luck, Roguejoker86 suggests the following:
"For the civilian on top of the temple I ran across the top scaring him. Since I didn't take out the droid he ran towards the corner of a ledge. Just wait it out and the civilian will then start to walk towards one of the corner structures. That'll be your chance to harvest him but watch out since going on top also spawns two more droids."

Also, I was playing on Easy mode for the harvesting. This helps dispose of enemies protecting civilians.

Aliens Vs Predator

Grim Reaper Guide

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When getting this i spent ages searching for a good guide and this is the best i found, (this is not my work it comes from the IGN website) it also has pictures so if you're using the guide above this might help.

https://www.ign.com/wikis/aliens-vs-predator/Infestation

hope it helps!

Edit. Updated the link, should work now.