To get the achievement it is necessary to have the enemy unit killed on the geyser tile (the flame going from the hole in the floor). The easiest way to do that is in Episode Seventeen in the very beginning: instead of turning off the valve, teleport with Marcus over the flames and "summon" an enemy to them (attract and flee), use shield ability to pass the flames and shoot (don't kill!) the enemy to weaken it in order to have a few hit points. The achievement will be unlocked, when the enemy dies in the flames.