Although alot of people say the snipers outside Ravenholm must be defeated that is NOT true.
I started the chapter "Follow Freeman" as a new game and killed only the snipers I saw in that chapter for this achievement to unlock.
So, start up "Follow Freeman" and find those blue lines. Blue line=Sinper, there should be a total of 5. Kill EVERY sniper you see BEFORE moving on, you wont be able to backtrack to kill the snipers if you move on.
You can use grenades to kill them, but I found the Secondary fire with the SMG alot easier.
If you find yourself running out of ammo you can use a cheat to refill it. But in order to get the secondary ammo you MUST have 1 secondary ammo left.
Also, make sure you are holding the gravity gun with the gun you need ammo for as your quickchange for this to work.
Refill Ammo Cheat: Y, B, A, X, RB, Y, X, A, B, RB.
Also if you are having trouble killing the snipers because you can't get a shot off and they keep hitting you there is an invincibility cheat that will keep the bullets from damaging you and give you time to line up your shot.
Invincibility Cheat: LB, UP, RB, UP, LB, LB, UP, RB, RB, UP.
Cheats will NOT disable achievements, so don't worry about using these.