This achievement may be tricky for some, but there is a super easy way to accomplish it.
Begin a Classic Casual match on any map, and buy a Desert Eagle at the start of the round. Let the Terrorist team plant the bomb and then proceed to the bomb site. Usually bots will just crouch in corners, allowing you to net an easy headshot on them. Just make sure you have shot all but one bullet, then aim for the head and done!
I ended up grabbing this by complete chance, so this is how it might work out for you as well.