How do I get the Headshot Honcho achievement in Halo 3?

[Your Worst Nightmare]

I kept shooting grunts in the head with the grunt birthday party skull and I know I got more then ten. Does anybody know how to get it?

 

[GamerBeast]

You need to get a headshot by using a sniper rifle ONLY. In order to get this achievement no other guns will count.

 

[?Jeremy?]

Do the mission were you start with the sniper and just take the brutes out cause they're easyer..