Last night I played some Left 4 Dead with some friends and I came out with some great achievements, that are quite difficult to get so I figured I should share them.

Achievements	Gamerscore
Man vs Tank Single-handedly kill a Tank.	30 Acquired 30/7/2009
Witch Hunter Kill a Witch without any Survivor taking damage from her.	20 Acquired 30/7/2009
Cr0wnd Kill a Witch with a single headshot.	30 Acquired 30/7/2009