This achievement is pretty unclear. Based on comments I've received, I would suggest you take some shots from an enemy in a CTF ranked game (let him shoot you a few times) as you're defending the flag, let him grab the flag and get some distance from you as he returns to base, and then get a headshot on him.
It's unlocked for sniper rifles and bows, and even from other weapons. If none of this works, my *original* solution below will explain what I originally did to get the achievement.Shadow Enz said:
This achievement unlocks when you immediately kill a flag carrier AND return the flag as soon as the flag is initially picked up. (Must be a ranked game)
If you do both of these things fast enough, the achievement unlocks. It must be done very quickly for this achievment to unlock.
This goes for any "Canadian" CTF achievement (ie - Timeshift).