This achievement is missable because there are no enemies after the story, and it only works in certain story sections. You need an enemy section with a bottle. The bottle can be picked up and crafted into a Molotov cocktail. Then, throw it at two enemies standing close together. A good place to do this at is the second enemy encounter in the game, shortly after escaping the big cave in Cozumel (Mission: Hunter's Moon, Region: Cozumel, Objective: Get through the Dig Site). There are three enemies standing around a forklift. On the forklift are some bottles. Hold RB to craft one into a Molotov, then throw it at the two enemies standing close together to get the "Playing With Fire" achievement.