Semper Fidelis Achievement
Complete the campaign story on Hard
Complete all the missions on Hard difficulty. Playing on hard, enemies seem to develop a 6th sense and they will notice you and lay lots of supressing fire. Take advantage of cover and prone, peek out to kill 1-2 enemies at a time and then shrink back into cover. It is also very easy to get killed (usually in 1-2 shots if relatively close), so if you start getting hit, quickly find cover.
Also, take advantage of any flanking routes/walls and abuse them if you want to get by missions quicker. This is similar to Call of Duty where you kind of have to force your squadmates through the mission. They are also invincible, so use that to your advantage.