This perk requires you to get 80 kills with the assault rifle while leaning (
) out of cover. This can easily be done within normal story progress, but if you have some kills left or want to get this out of yor way as fast as possible, there is a good spot on chapter 2 - asylum
After you cleared the floor you started on, use the staircase to go down one floor. Kill the remaining bad guys and proceed into the room, where you have to push a button to open some doors. Crouch immediately and lean up to kill the fist guy. Stay crouched and leave the room. Go right immediately after the doorway and crouch behind the little bookshelf. Lean up and kill everything that comes towards you. Sometimes, the second wave needs to be triggered by leaving your cover and run a little bit towards the hall, go back behind your cover and clear the area.
You can get 7 kills here within a minute. Reloading the checkpoint will set you back just before you have to push the button, rinse and repeat.