This is story related and unmissable. You will get it after completing mission 9 in Far Cry 3.
It is quite straightforward and no real tricks to it, other than remembering to shoot water pipes.
You will have to make your way through an enemy camp. There is a target with a yellow marker above his head at the furthest end of the camp (make sure to use your camera to mark all enemies to find him). Once you take him out, loot the body for the key to the entrance of a cave just next to him. Make your way through the cave, and at the end of it you will be caught.
There will be a cutscene which will end with Vaas setting the room you and Liza are in on Fire. Make your way through the building until you find her, shooting the pipes to put out the fires in your way. There is a timer so keep an eye on it. Once you find Liza, you will end up in a car where enemies will chase you. You have to shoot them all down. At the end of the sequence, once you have escaped, the trophy will pop.
For a video guide, see this link :
All video credit goes to baseball4evPC via Youtube