Achievement: The Professional
This achievements is in the mission "Comrades". As soon as you go out on the stairs outside chasing the target, that is when the timer begins. This is very easy on Normal, so I can't imagine how easy it is on Easy, haha. Basically, never take cover, always run-and-gun. I suggest using the Ak-74 you start out with in the beginning of the mission, no point using the silenced weapon since you are making yourself overt to the enemy, and the fact that the silenced gun (still can't figure out the name of the gun, haha) only has twenty-one bullets; the 74 has thirty-one bullets. Rush down the street to the guy until you reach the cutscene where **SPOILER** one of your teammates are blown up. Afterwards, going down the small tunnel to the stairs, you should unlock the achievement now, so if you didn't unlock it, you must restart the whole mission again. I honestly got it the first "try", even though I didn't even know this achievement existed upon getting it, haha. So remember; DO NOT DIE, and don't just gun it completely. When you enter the street, you will come across police, kill them quickly and then run-and-gun with the '74!
Please leave comments if I need to correct anything.