This achievement is extremely easy and can be done in a custom game with another friend and/or I'm guessing a second controller.
I did this with a friend. Go into Multiplayer > Custon Games > Halo 2 > Anniversary > map doesn't matter and for the game type I would go Snipers just so you have a sniper to start with.
Meet up with your friend in the middle of the map and have them zoom in with their sniper rifles. Then just shoot them in the head and the achievement will pop!