Kill a demon with fire.

You can find an easy to kill demon while you are looking for the teddy bear. See the description for "Friend of the Crew" for a location. Before running in however, note that this demon is on the top of a structure and is resting in its nest. You can approach silently and when you reach the top, assuming you haven't been detected, you can throw a Molotov at it to kill it before it can react to you. Make a quick save at the bottom of the structure before attempting this on the off chance you get spotted as trying to kill a flying demon with a Molotov will be almost impossible.

There are also incendiary rounds you can use in the Tikhar however they aren't immediately available and you will need good aim to bring down a flying or otherwise aggravated demon so the area detailed above is by far the easiest place to unlock this achievement.