It is highly recommended to attempt this during New Game+ mode, which is unlocked after completing the game. You can then select "New Game+" at the main menu, and load Chapter 10 at the chapter selection screen. You will automatically start with the rocket launcher, which will make this significantly easier. The part for this trophy starts when you reach the crematorium where everything is burning. Before this point, you cannot kill Laura (she will always flee when she is about to die and regenerate her health -- so just ignore her until you get to this point). Use the fire traps and corpses to burn her. While she is burning, you can attack with the rocket launcher. It takes approximately ten direct hits with the rocket launcher to kill her and get the "Bathed In Flames" trophy.