First, finish the game in any difficulty (even easy.)
Then, from the main menu go to the options menu and change the difficulty to Hard (just in case change any onther option too) and apply changes.
Now, load/resume your game.
When the game is loaded (the last part of the game) press the options buttom and go to the options menu again and make another change (I turned subtitles off) and apply the changes again.
Finally, finish the game once again and....TROPHY WON!