The Flamethrower (Tech) skill is required to get the "Pyrotechnics Expert" achievement. Lighting three enemies on fire with one continuous flamethrower attack works best with enemy dropships. Wait under a dropship and when the enemies jump out, immediately use the flamethrower on them. You encounter a lot of dropships throughout the game. This can be done on the very first dropship in "Chapter 1: Planetfall" (Habitat 7). Every drop ship has at least a few enemies jumping off it at the same time. Make sure to set the difficulty to Normal or higher or else the enemies will die before they start to burn. You can also do this where enemies are standing close together so you can hit them all before the flamethrower runs out.