To complete Metro Exodus 100% and unlock the Platinum Trophy, you must unlock all trophies/achievements. In this guide we explain the following task to you:
Kill a demon with fire.
An overview of all trophies and achievements can be found here.
Kill a demon with fire
To receive the Firebird Trophy, you must kill a demon with fire. In Chapter 3 (Volga), a demon is hidden in a nest that you can easily attack and kill with Molotov cocktails.