BioShock Infinite

For this achievement you'll need to acquire a Bird's Eye Sniper Rifle, it can be acquired from Elizabeth at the Hall of Heroes plaza when your being shot at my enemy snipers. Just shoot and kill 30 enemies with it and you'll pop this 5G achievement!

Note 1: You can re-load checkpoints if you want to get this achievement fairly quicker than usual, as the kills will still progress.

Note 2: Your kill tracking for this achievement will carry across multiple playthroughs if you don't manage to get it right away.

If you, like me, suck with the sniper rifle and barely used it the whole game, the best place to do this is at the final boss battle, Command Deck, right at the beggining just take the rail to the top part of the ship, and there will be a barrel with a sniper rifle on it, just take it and aim for the enemies down at the deck, they will be focused at the ship's core so you can take your sweet time aiming at them! if you run out of enemies just reload the checkpoint!

Bioshock Infinite - Sniper kills Farming guide

The video guide below shows where this achievement can easily be obtained with hassle and also 5 other achievements. All the achievements are in the same area.

Please enjoy

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!