How to unlock: You have to set three enemies on fire with one Flamethrower attack (single-player mode).
Commentary: Make your attempts in small rooms and/or while fighting numerous enemy groups. This increases your chance to ignite three foes simultaneously.
The easiest way to acquire this trophy is to drive around the surface of the planet, from one enemy outpost to another, looking for a place in which enemy reinforcements appear. Once a dropship appears in the area, quickly rush towards it, get out of the NOMAD and set at least 3 enemies on fire with the Flamethrower ability.
If you didn't get the trophy, check if the enemies weren't protected by shields and/or armors - if that's the case, repeat the whole process and target enemies that only have a health (red) bar.