Credit to "Sellers", the original writer of the guide.
For this achievement you need to scan all enemies at least once with the Genesis. This is missable as there are some enemies that you only encounter once. Some enemies can be scanned multiple times but this is not required, just actively scan any new enemy you come across and you won't have to worry about this, your progress carries over to subsequent playthroughs if you happen to miss any. Unfortunately, there is no way to track which ones you have scanned in game.
List of enemies below:
Standard Ooze - Very common throughout the game
Pincer Ooze - Large claw arms
Tricorne Ooze - Fire projectiles at you from a distance
Chunk Ooze - Blobs that explode when you get close
Globster - Blobs you encounter on the beach
Wall Blister - Pods stuck to walls/cieling
Fenrir - Small and large wolves, scan both
Scagdead - Fat with a saw hand, a few of these in the game
Rachael Ooze - You will encounter her a few times
Ghiozzo - Infected fish
Sea Creeper - Siren/sea creature like enemies, encountered mainly in the Bilge
Hunter - Large reptile like enemies
Farfarello - Hunters than turn invisible
Draghignizzo - Large rock like boss, only fought on the observation deck
Scarmiglione - Sword and shield weidlers, scan when it's just a pair of legs to be safe
Malacoda - Giant tentacle boss in episode 11, only fought once
Abyss Norman - The last boss
*Taken from playstationtrophies.org