Left 4 Dead

This is fairly inevitable. The best tools for this are, surprisingly, not the shotguns. Using the pistols, assault rifles or submachine guns keep your aim level and sweep back and forth at about head level when they charge. You can accrue more headshots this way.

(I actually did this primarily with the auto shotgun, but I noticed I got more headshots when I used the assault rifle.)

This achievement is cumulative through all your campaign, vs. & survival playthroughs.

If you play the game enough, you'll get this on accident, but there are ways to make headshots easier. At the beginning of levels where a sniper rifle is available in the saferoom, such as Death Toll 4, take it and pop as many zombie heads as you can before leaving the safe area.

The quickest way I found, though, is to grab a pipe bomb and an uzi or assault rifle. Throw the pipe bomb, and shoot at the clump of zombies that forms around it at head level. You'll get a bunch of quick headshots that way.

what i would do i get a mini Uzi an go on the first campaign on easy then grab a automatic rifle. aim up word an whenever you get a pipe bomb throw it an spray the crowd to the head you should have the achievement within the first time or the second for sure.