Mindjack

A good place I found was on 7-0: Revealed.

You run through a corridor, pick up some guns, shoot out the Quick Draw in front of you, mindjack it, and shoot the Icarus soldiers in front of you (3 of them). After you do that, another 3 - 5 come out. If your Quick Draw blows up, there is another one past the first 3 soldiers (behind some metal crates, but you cannot miss it, it is on the main path to the next check point, shooting at you). If you do not get enough kills, just restart. I restarted twice and got the achievement.

The quick draw first appears in the Colosseum level shortly after you have killed the titan in the same area. It appears next to a doorway next to the escalators / stairs right after a new wave of enemies appears. Get into cover right next to the barricade where it spawns and throw and couple of grenades at it to take it down then mind hack it. Then drop out of the character you are using and hop in, then unleash your guns on the enemies. Every time you use the quick draw this way you'll get about 2 - 5 kills, so on average it'll take you 3 reloads of this level to get this achievement out of the way.