Left 4 Dead

Go to No Mercy, rooftop finale.. Once you exit the safehouse, go up to the top of the stairs and throw a grenade (not molotov), go back to safehouse and then back up stairs. You should now have a boomer and hunter coming down the hallway. Go back to safehouse and shut door. Once hunter comes down to the door, back all the way up, and he will start jumping. line that shot up, and get that cheevo!!

I tired out a couple of methods, but the one that worked for me was posted on youtube and goes as follows:

Run through Dead Air, chapter 3 on easy with 3 bots to the safe room at the end. Instead of going in and closing the door, however, grab the M-16, heal, and/or reload as needed and go back outside to the long hallway. All the bots will wait in the safe room.

Now camp about 1/2 way down the hallway. Every 15 seconds or so a special infected will spawn at the end of the hallway. If it's a boomer or a smoker, just kill it, but if it's a hunter just line up your shot and fire bursts over his head when you think he's going to jump. If you are at the right distance you get two jumps, and if you screw it up you'll get a new hunter in about a minute... it took me three to get the headshot.

The best things about this method are: 1) You have time to line up your shots, and he comes straight at you so it's easy, 2) If you get pounced the bots will run out, save you, then go back in the saferoom, so they never steal your kills, 3) If you really screw up you can dip into the safe room for additional ammo and health packs then go right back out to continue camping, and 4) Endless targets in rapid succession but NO normal infected or tanks EDIT: Do'h, apparently you can get normal infected, but not that many (relatively speaking).

Pure win!

When you see a hunter, he's gonna jump. Chances are at you if you're the pointman.

Aim slightly above his head and fire quick shots or short bursts. Either you have great reflexes or will just get it luckily (over time like me)

Recommend dual pistols, uzi or machine gun.

* basically same as melee achievment *

Just thought I would add a video from Rooster Teeth which shows another way to get this achievement.

It is not foolproof but it worked for me after trying lots of other methods.

The easiest way i found to get this on is not the no mercy finale, but No Mercy Chapter 2.
If you have 2 controllers, bring your second guy out of the saferoom. A hunter will spawn almost instantly. Run back to the safe room but leave the door OPEN. let the hunter pounce you and switch back to your 1st controller. Shut the door on the hunter so he is trapped in the safe room. If you then open the door it will glitch him into the corner. The hunter will then attempt to move around the door, just keep clubbing him into the corner. Eventually he will attempt to jump over the door, where if you shoot just below the ceiling you will headshot him.
After several hours of unsuccessfully attempting Chapter 5, i managed this 1st try.

I tried so many different methods of getting this acheivment and, i really didnt think this one would work but it did!

You may find you buddys kill a lot of the special infected but just give it a while and you will get a good shot

Make sure that all other Survivors are dead. On the last part of No Mercy. Go up the staircase and you'll see a few zombies. Grab a pipe bomb and throw it. This should kill most of the normal zombies.

You should hear or if you have the dialog on see a Hunter and a Boomer. Don't worry about the Boomer. Just run back to the Safe Room and shut the door. Grab a Sniper Rifle. Now the Hunter should be lunging at you. But, the door is in the way. This is based on luck. When you think the Hunter has jumped, shoot the Sniper Rifle in it's direction. Hopefully, it hit the Hunter while he was in the air. It doesn't necessarily have to a headshot. You just have to hit it. If the Sniper Rifle isn't working, try the shotgun or machine gun.

I found that the easiest way is to get on a rooftop or higher ground. A great place for this is the survival mode DLC. You can just stand on top of the roof. Although there is no 100% chance of this happening anywhere, this is a good place to start. If the hunter pounces at you (Probably wont reach on the first time since your on the rooftop)and you shoot down, you can get a headshot much easiersince thats the thing closest to you. I also suggest using pistols for this.

This achievment requires a great amount of luck, so if this does not work try one of the other solutions.

i got this with dual pistols, in order to do this you have to be quick and accurate with your shots. you can do this anywhere but i suggest NOT doing this in the subway on the No Mercy campaign. mostly because of the small environment that your in. i suggest doing this on the Blood Harvest campaign, because of the nice open fields.

i did this with 2 other people the best way i found was play vs one person infected other 2 survivours.kill the 2 bots then the person who wants the achievement stands in a long room and the infected (as a hunter) crouches at the other end. then the hunter pounces towards the person wanting the achievement and the survivour trys to shoot him in the head. if the survivour misses then you have your buddy punch the hunter off and try again.