LEVEL A CHARGE achievement in Left 4 Dead 2

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.