The “bestiary” is the list of all the creatures you’ve encountered during the game. Which ones you find for this achievement doesn't matter but once you find half (76/153) of them this achievement will pop. This is about as close to story related as you can get without being story related... naturally this should pop around half way through your play through. If you wish to “grind” this, than get to as many different locations as possible so the spread of possible enemies is wider. Repeating areas will repeat enemies you’ve already engaged. New area will mean new enemies. Some enemies are rare and only pop under certain circumstances, like the airship pirates for example. Not only is this event uncommon but spawning the pirate captain can be very hard.