In Chapter 3, Fire Sale, The Assualt, when you're going through the building with Rianna, you will come to a part where you are looking at a wide open lot with a plenty of bad guys. After Connor uses the truck to distract them, Hopper will then launch the white phosphorous which lights most enemies on fire. You will then have a sniper to use and there will be enemies to the left that is not on fire. Just do not shoot the guys on fire or do not shoot at all and you will get the achievement. It will unlock after they ask Hopper for the second round. You can snipe the enemies to the left that is not on fire if you want.
NOTE: To the right of the balcony, there will be a news pick-up.
Thanks to HiradC for the news pickup info.