Fracture

You will get your first taste of the Scorpion Sniper Rifle near the very end of Act 2 in the area where you are to rescue the prisoner. Grab the rifle and headshot every enemy in this area.

I then suggest reloading from the last checkpoint (or dying) as there is a save point just before you grab the rifle. The kills stack even after a death or reload so the 10 kills you may have just gotten will become 20 after the reload (given you kill 10 more enemies). Just keep reloading after you've used up all of the ammo and you'll have the achievement in no time.

You can also do this in the weapons test, although you will need 80 data cells to unlock all weapons and 100 data cells to unlock the tdv in weapons test mode. All you need to do is equip the weapon and step on the red circle for a little and that will spawn enemies...kill the enemies with the weapon, it will unlock after you get 25 kills with the scorpion, it is located on top of the building that you spawn in.

I actually got this achievement while playing with a friend on the multiplayer and i was using the scorpion sniper rifle at a map called solar winds or something similar to that i went near this small wall and found a scorpion sniper rifle and killed my friend i would say a couple times but i have killed my friends on the multiplayer before a lot and before i knew it the achievement unlocked so you can also try doing this in the multiplayer or you can also do it in the single player...