This is a rather easy achievement, albeit time consuming. All you have to do is kill 300 monsters with fire. Everything that can set monsters on fire counts: Barrels, molotovs, the flame thrower, Nathan's boosted flashlight etc.
Since barrels and molotovs are available so soon, you can start working on getting the achievement at the very start of the game. Beware when co-op:ing though that fire hurts both you and your co-op partner so try not to blast away in your partners direction too much. The damage from fire is negligable though when compared to the damage from monsters on the tougher difficulties.
Note that it's not enough to just set the monster on fire and then kill it, they have to die FROM the fire. Which makes the flamethrower the most reliable way to actually kill them WITH fire.
Playing as Sean or Nathan for this achievement is recommended for Sean's no-reload-necessary skill and Nathan's high explosives and boosted flashlight skills. I'm fairly sure that the counter stacks between characters, if anyone else could confirm that it'd be great.Weapon locations: