Here is another trophy where it is near impossible to miss. You will be taking many long-shots on enemies throughout the campaign. So if you plan on actually completing the campaign, you don't have to worry about ever grinding for this.
There isn't one single spot that you will accrue all these kills at. Literally just keep playing the campaign and this will unlock when you least expect it.
You can check to see how far your kills are (before you pull the trigger) by checking the distance on the enemy when you look through your scope.