Singularity

As mentioned by Cataclysmic V, most players will obtain this in a single run-through. One method of obtaining this achievement early is to get the the point early in the game where you are defending your bunker from enemy attacks (where you first recieve the sniper rifle). Make use of all of the spare ammo in the bunker and mow down as many incoming enemies as possible with the 40 AR9, rather than sniping them. You will also find it easier to achieve if you place your weapon upgrades on the 40 AR9, especially when playing on the harder difficulties.

A good grinding spot is towards the end of the first mission. At one point, you'll have to fend off Russian soldiers outside a building with your computer AI Delvin(?). Kill as many soldiers till the soldiers start entering the building through the windows. Get behind cover and rack up as many kills as you can. Make sure not to exit the room. Some point you'll die, so just rinse and repeat. This can also help towards the other guns(AR, Shotgun, Sniper, and the Centurion Pistol).