Quantum Theory

the easiest way to get this is on the second mission when you have to storm the gates of the first tower. try to find/have a sniper rifle with you at this point.
from the start point, directly in front of you should be some turrets located next to the gate you are supposed to storm. simply shoot the farthest of these and you should get your achievement for this :)

quote from bill 13: Works perfect, got it on my first try. The only thing I would add which I assume you must do is play through the first chapter again and make sure you pickup the Plasma Lancer. It's towards the end of the level when you have the battle with the largest concentration of enemies so far and it's on the ground off to the left.

So I found a slightly easier way to get this. At the start of chapter/scene 7 you'll be attacked by some enemies. One of the enemies is on a raised Walk-way that's above and in front of you. Also Infront of you on a piece of cover is a plasma launcher. Grab it and turn around, and go into the Left corner by where you started the level. Slowly move up and hug the wall to your right until you can just see the head of the enemy sniper. Set your sights on the enemys head and proceed to blow it away. The achievement should unlock for you.