All you have to do is get 200 kills with an AR (Assault Rifle) in the campaign. Here's a list of the assault rifles in the game:
AK-47
AK-74
AUG A1
FN-FAL
G36
G3A4
M16A2
These are very common, I found the AK47 and AK74 to be the most commonly dropped by the enemies, and even if you don't try to get this achievement, you should naturally get it on your own!
(My first achievement guide lol aye)