Left 4 Dead 2

Here's a real easy way to do it in versus by yourself and a guest. Make sure the guest is infected and you are a survivor. Kill all the AI first. Then keep killing the guest until they are a charger. Shoot the charger while he is alive in the leg with a pistol until they are down to about 4 health. Then set up the charger to charge, then stand behind the charger with your melee weapon of choice. Pull both right triggers at the same time and you should get it. Hope I helped everyone.

UPDATE: try this on THE PARISH - THE BRIDGE just keep jumping off the bridge untill you get the charger.. and then follow procedure above^^..(just dont let the bridge down)
Credit goes to - VETTE BOM NL (thanks).

You can get lucky and try this in the campaign. If you are looking to go this route, keep these things in mind.
Any melee weapon will do.
The charger spawns on every level so there is no specific place that is best for this.
If he is coming straight at you, you will have a very hard time doing this, so when you see him start to charge, swing out to the side and try to melee him.
One hit may not do it, so most times you may not get this even if you manage to hit him while he is charging, but if you have AI or partner controlled teamates, they can put a round or two in him before you hit him.
If your going to soften him up, best to use an uzi or pistol, only a few bullets each will be enough to make a melee fatal.

If you wanna make this really easy on yourself, go into versus with one friend, kill all your AI teamates, have your friend continue to spawn and be killed until he is the Charger. Have your friend aim his charge slightly to the side of you and melee him as he runs past. Have him aim at you so his targeting reticle is slightly past you so he is close enough for you to strike.

credit goes to machmota9 i just put it his words into video form I think this is the easiest way of getting this

[Re-posted from site failure, His post isn't here right now but it may be soon]

A way i found to get the achievement was that i loaded up the finale for dark carnival, had another controller be the charger (my other xbl account) and i kept letting the AI kill me until i was a charger for my other account. With the account i was trying to get the achievement with i ran the the stage, found a melee weapon and ran back to the safe room, but before you run inside it, with your other controller, spawn in the back of the safe room, around the corner, the AI's will tend to stay on the outside of the safe room just killing other zombies or special infected, now with you main account, shoot the charger until he has very low health then get directly behind him and pull both right triggers at the same time, as the people above me have talked about this worked perfectly for me, the AI's kept the special infected out and zombies.

This is based off of machmota9's method of how to get the achievement I've put it into a video if that makes it any easier to follow. The video is pretty self explanatory, hope this helps.

Here's how I did it in Single Player on command.

1. Fire up The Passing on Normal.
2. Get any melee weapon; doesn't matter.
3. Sometime shortly before or in the Safe Room, grab a sniper rifle.
4. Physically get into the safe room, but don't close the door.
5. Now just hover in the doorway and listen for Special Infected.
6. A Charger will eventually show up down the wide open street.
6.5 (Note: Other SI will show up, as well as hordes, so be wary.)
7. Hit him exactly 4 times with the Sniper Rifle.
8. Switch to your melee weapon and wait for him to charge down the street towards you.
9. Swing away! Ding! *Achievement Unlocked*
10. All the while, the bots just sit in the Safe Room and do nothing.

-Jess

The only other thing that I can think of to do it legitimately is to have Adrenaline on hand and a sharp melee weapon like the Kitana, the Axe, or the Machette and take the adrenaline when you see the Charger and just try to nail him while he's chargin; technically he's still charging when he grabs someone so you can use that to your advantage.

Try to wait for him in an open area so you'll have time to see him coming.

Lately, the two controller method hasn't been working because of server problems (for me anyways) so if you're having the same problem as me heres another method of getting it.

You will need Adrenaline, a melee weapon (chainsaw would be preferable but I did it with a baseball bat. It works with any melee weapon really,)
NOTE: Turn full captions on
Load up Dead Center- The streets and rush to the part where you have to cross a longpathway. stay a couple of feet away from the stairs and wait until you see a charger. IMMEDIATELY take your adrenaline and wait until he pops up in your face. Your AI will be shooting him while he's coming up the stairs, decreasing his health so you can easily kill him. When the captions warn you that the charger is about to charge swing your melee weapon and the achievement will pop up. You have to hit him RIGHT when he's about to charge or else he'll run through you and start pummeling you.

On the youtube video skip to 7:06 if you don't understand where you have to be when getting the achievement. I do not own this video.