After you jumped out of the window in the burning building in
chapter 6 you will see a big gate at the end of the narrow path. Once you get close to it, enemies will burst out. Step back a bit and crouch behind the crate that's leaning against the wall.
This counts as cover and you can stick your head out holding
and shoot all enemies. Go on and use the upcoming corners as covers. This section is filled with enemies and gets you 50 kills behind cover in no time.
Reload the checkpoint after a few corners and you will start at the gate again.