Set up a multiplayer match of Deathmatch with max novice bots. Put on the mutator "Weapon Replacement" and edit it and set all the guns to Sniper Rifles. Set the kill limit to 20 and the map to Rising Sun. Then just kill the bots 20 times with a headshot. Do this 9 more times for the achievement. Note: You get the award when you get 15 kills but I suggest you do it to 20 kills so you can get "Paint the town red" and other achievements quicker. Also you can't get these in Instant Action, so remember that!
Alternately, you can use the Morbias map against one Novice bot with SloMo and Weapon Replacement mutators turned on and force respawn set to "Yes". If you stand near the middle, the bot will often spawn down the same tunnel for most of the match and you can easily get 20 kills in less than five minutes. The SloMo makes it easier to hit the headshot if he insists on moving.
This video explains how to setup the match and use the Weapon Replacement mutator. Do this for every other achievement, choosing whatever weapon you need for the achievement.