Metro 2033

I was able to do this very easily on "Outpost" using a sniper scope weapon. I died a couple of times, but the checkpoints are nicely placed to break it up if you die.

(NOTE: If you don't have the silent scoped sniper weapon, load the level "Child" on Easy (the one just before "Outpost"), and run through the entire level (takes only a minute or two). You don't need to shoot anything. At the end of the level, there is a vendor who sells the silent scoped sniper. )

Details:
- On 'Easy' difficulty start "Outpost" and use your scoped sniper
- At the start, from the safety of the tunnel exit take out the single guard walking back and forth above the initial area
- Remain in/near the tunnel and take out the 6 guards who come down in a patrol
- Leave the area by running up stairs and there are two guys you can take out by a hut - you'll have to be quick but it's pretty easy to headshot them without using the scope at close range. Walk around the back of the hut (watch out for the trap) to trigger a checkpoint
- Make your way to the "main" building with the crane next to it. By standing outside on the crane, you can take out all the guys (about 5) in the building through the window
- Proceed into the building, and through to the exposed area where the demon often lands, and using the stairs to your right, go down to the lower level (with a checkpoint on the way)
- On the lowest floor, run down stairs, turn immediately to your left, and follow the hall and enter the last room on your left. Take out the guy here and camp the room until 2 or 3 other guards come .
- Peek back into the hallway and take out the remaining 2 or so guards
- Exit the building, triggering a checkpoint
- Snipe the 6 or 7 guys outside while proceeding through the cars.

There are over 25 guys on this level alone, so you can miss one or two, but like me you may well have some headshots from earlier in your save and have it unlock before hitting 25 on this run (I got it on my 22nd guy for example).

Easyest way is to Use The AK with Sniper Scope and play the level "The Depot" on Easy! Aim for the enemys Heads (Humans only) and you should get it very quickly! There are enough enemys walking around you can kill and you still have the option to Reload a Level if you not reach 25 Headshots at first try! Hope this helps... (Excuse my bad English)

From how far into the game I unlocked this achievement, I believe that it's only headshots on humans that count towards this achievement. This should be easily obtainable, there are many missions where you are surrounded by hostile humans. Just remember to get a weapon or two that has a scope on it, this will increase your accuracy tenfold.

This achievement can be done with every gun, just make sure you
make headshots with it, i did them with my (silenced)pistol and
riffle. So you don't need a sniper to-do this Achievement (it is even
possible to obtain this achievement by preforming headshots with a
shotgun, so it couldn't be any easier).

Have fun hunting

An even easier way to get this achievement is to do something similar to the Air Gunner achievement method. Load up the level "Armory". After you talk to Andrew the Blacksmith, buy the "Tihar Sniper with Scope" from the gun vendor on the wall, then buy a bunch of Ball Bearing ammo from the vendor next to him. The vendors are in the same area that the people ask you to "jump in the hole". If you jump in the hole, you have gone to far and need to restart, as you will not be able to buy the sniper.

Proceed through the level to get the "Frontlines" level, it should only take you about 2 minutes to get there. Make sure your Pneumatic Sniper is pumped up to the max, then just simply pick off each enemy with a headshot. It might take you 10-20 minutes if you are careful, but you're pretty much guaranteed to get the achievement by the end, as long as you make sure you are getting headshots on each enemy. My advice is to keep your distance and just pick each enemy off, it's not too challenging.

If you want, you can count the amount of headshots as you go just to keep yourself on track. If you for some reason don't get the 25 by the end, just reload the level. There are at least 40 people in this level if not more. Should be easy enough, I just got it in about 15 minutes total.

Note: As mentioned above, it appears that all 25 headshots must be on humans.