There are many opportunities to obtain this during Chapter V (5). First, you will need a second weapon with a zoom-in scope (Nucleon Charge Rifle). A good opportunity is mid-way through the chapter. You will come to an area where you pull a lever to move a giant bot to make a bridge. After pulling it a handful of Sniper Bots will come out. Good way to do this is to lock onto a Sniper Bot and wait for them to shoot, once they have shot, wait for them to dismount and be in the area. Quickly shoot to kill the Sniper Bot and unlock the achievement.
Here is a great video guide by Kal_El showing you a great spot to obtain this.