This Achievement is to get 50% of all the beast in the beastiary in the game. It should be fairly simple. Most enemies are not miss-able but there is a chunk that are. I used this guide on Steam. This is not my guide, all the credit goes to Biggs.
https://steamcommunity.com/sharedfiles/filedetails/?id=26422...
WARNING!!!! The Following Beasts are Miss-able
Floating Continent (inner sea) - Must be defeated before Saronia
#031: Killer Fish
#032: Hermit
#033: Sea Elemental
#034: Tangie
#035: Sahagin
Castle Hein - Must be defeated before completing the dungeon
#066: Pharaoh
#067: Lemur
#068: Lamia
#069: Demon
#070: Dullahan
Floating Continent (outer sea) - Must be defeated before Saronia
#071: Anet
#072: Mermaid
#073: Seahorse
#074: Sea Serpant
Surface World (sea) - Make sure to sail in the ocean on the Surface after defeating the Kraken
#112 - Sea Dragon
Lake Dohr - These monster cannot be encountered after Leviathan is defeated and the player leaves Lake Dohr
#140: Plancti
#141: Sea Lion
#142: Remora
Doga's Grotto - These monster cannot be encountered after the player defeats the boss and leaves Doga's Grotto
#156: Ogre
#157: Cyclops
#158: Nemesis
#159: Humbaba