Metro 2033

I got this literally just after I got the DJ Artyom achievement (Outpost level).

If you wait inside that little hut with the transmitter the two demons that are flying around will land directly outside it for about 3 seconds before flying off again. You can shoot them if you want, but I just threw a sticky grenade as soon as they landed.

I actually got this while I was doing the RANGER achievment at Dead City 1 and Dead City 2. During the RANGER walkthrough, you should encounter 2 sticky bombs (or alternatively you can buy them at The Market which is just before Dead City 1).

The 1st Demon you will encounter at Dead City 1, outside, by the stairs. He will be facing away and just sitting & chilling, so just go up close and throw a sticky bomb at him.

The 2nd Demon is at Dead City 2, right after you meet up with your friend, you will encounter the Demon who will fly away, just wait until he flies around and he'll land right in front of you for a few seconds, throw another sticky bomb (note, this is BEFORE you face the swamp of monters)

I'm surprised no one has mentioned this yet. The other guides are probably fine, but by far the easiest way is to kill the one on the last chapter, but one thing at a time.

First things first, I have confirmed, as with pretty much all of the achievements in the game, this needs to be done in one playthrough. Reloading a checkpoint to kill the same one will not count.

The first one you'll want to kill is in Dead City 1. This one I know has been mentioned, but it really is your best option. After making your way through a couple of buildings and fighting a pack of Watchers, you'll cross the water and go up a staircase. At the top of the stairs you'll see a demon feeding on a dead body (you can't miss it). If you have purchased the DLC, you'll want to use the volt driver and shoot it from here. I took it out in one shot using this. This can be found in Lost Tunnels after killing all the bandits. You'll find it lying next to a plethora of ammo near the center of the room. If you don't have the DLC, you can try and land a sticky grenade which you can buy in the previous level, Market. If you don't have any grenades, you can try the risky way. Running past the demon will send him into flight, but it is necessary if you need the weapon. Go down the stairs and right over the wall. At the back of the room you'll find a dead body with a Helsing and some arrows. Make sure it is primed, and when the demon lands to try and attack you, shoot the heck out of him. Ideally you won't have to do this, but it's an option if you need it.

After you've killed that one, tuck the achievement into the back of your mind until Top (The second level of chapter 7). Why wait this long? Because in this level, you actually have to kill one as part of the story. Once you catch up to Miller, a demon will come crashing through the wall and attack him. The game goes into slow motion and expects you to kill it. He is literally 2 meters or so in front of you, won't fly away, won't attack you, and you can use military grade ammo (since there are no more options to spend it). This one is very easy to kill and does count towards the achievement, because I popped it as soon as I killed it.