This is a quite easy achievement to get. Simply load up chapter 11 "Follow Freeman" and take out the snipers using grenades or RPG. One thing to keep in mind is not to proceed to quickly, as the snipers will despawn if you go too far.
I recommend making saves and loading them in case you run out of grenades.
The first two snipers are at the part where you are at street level. Take them out with grenades
before entering the basement, they will despawn after that point.
Proceed through the building until you make it to the third sniper who is shooting at the zombies. There is a box with infinite grenades here. You must take out this sniper before going up to the floor where Barney is, or the sniper will despawn. Restock on grenades before going up there or you may have to go down again.
The final two snipers are the ones pinning down Barney. Simply take them out with the grenades you have acquired and the achievement will pop.
If you wish images to assist you, take a look at this guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=14655...