This will quite likely be your first trophy in this game. To unlock this you'll need to defeat 9 of any 1 kind of monster and then travel to the Royal Library and go into the Seal Control Room. Once there select "Create Sealbook" and it will show you all the enemies you have defeated and how many of each you have stored in your Libra Book.
Any enemy where you have defeated at least 9 will display in white text which means you can create a sealbook of that monster. Press
, select Yes at the prompt to confirm and press
again to create your first sealbook and unlock the trophy. The first enemy you're likely to be able to get this with is the Hola-Orange which is the most common enemy in the first dungeon, Nameless Cave.
If there is any way that I can make this solution better please don't hesitate to let me know.