Another very good place that you can rack up lots of sniper rifle kills is right at the end of the 'Emporia' level. You'll be having a battle in front of Comstock House and the boss will spawn enemies constantly until it is down. There is a very handy tear here which gives you a sniper rifle, just in case you don't have one. Then simply crouch behind the cover near said tear, and kill all the spawning enemies you can until the cheevo pops.
So you won't have to worry about running out of enemies, and there's also vending machines nearby to keep stocked up on ammo if you run low (or if Elizabeth isn't finding any for you) I got quite a lot of sniper kills here so it worked like a treat. Hope it helps for some!