You will get this Platinum trophy after obtaining every other trophy in the game!
1. Complete the Story
2. Obtain all Collectibles (8 Statuettes, 20 Pieces of Subject 16's Video, 10 View Points, 100 Feathers and Scan 1 Glyph)
3. Purchase everything needed in the stronghold, clothes and weapons
4. Clean up all remaning trophies, these may already be obtained, like sprint for 100 meters or pickpocket 1000 Florins
There are guides for all trophis on TrueTrophies which will help you with all of these trophies!