For this trophy you will need to obtain 5 Silent Assassin Ratings under hardest difficulty of the game – Professional Mode. The trophy will pop upon completing your 5th mission with a Silent Assassin Rating. Also note that this trophy will also unlock 5 Normal Silent Assassins and 5 Expert Silent Assassins because the difficulty trophies stack. If you are intent on Platinuming this game, then this trophy is therefore unmissable because you have to a Silent Assassin Rating in every mission in order to unlock hidden weapons for the weapon related trophies. Lastly, you do not have to get the rating in consecutive missions.
See Silent Assassin for tips and conditions on how to get the rating. Also see the Professional Difficulty, Silent Assassin and Weapons Guide for a written/video guide on how to get this rating on all levels.