once you can remove the barricades with the rope wrapped up on it, you will be able to progress in the story in the Soviet Installation and will learn how to use Bottles plus one Cloth to make a Molotov. right afterwards there should be a group of enemies. i just picked up a Bottle with
to light it, got them to gather around me, dodged away with
, and threw the Bottle when they were piled up, by holding
to aim and pressing
not sure why Lara just figured this out, since she could already do the same thing with Gasoline Cans. i don't believe the achievement will trigger if you use those.