Due to the online feature of this game being all but dead you may need to find a boosting partner, you can create a gaming session here;
http://bit.ly/gTa5Tf, you'll need a total of 2 players to boost this game.
First of all, this achievement is accumulative, you don't need to go for this in a single match, it'll track your progress across your games, however, if you're boosting it doesn't take too long to get this achievement in a single game.
Now despite the achievement description stating you can use any weapon and get this achievement, you can't, a few select weapons are excluded from this achievement because they're not able to score a headshot, weapons such as the rocket launcher, the crossbow, grenades. When going for this achievement you'll know if you've scored a headshot because the game will tell you, on screen where it shows you who killed who there will be a little symbol.
Lastly, if you've decided to go for this in a single game it's a good idea to set the score limit at 35, that way if you accidentally mess up a kill you can still get the achievement in a single game.
As for which map to use, it's entire your choice, I personally opted to play "Rooftop Excess" due to it being a very small map.