It's notoriously hard to obtain due to enemies having variants or only showing up in one room.
See the Bestiary page for a complete list of enemies needed for this achievement.
Commonly Missed Enemies
Some common enemies missed are:
- Beholder Sentry - The drones spawned during the last boss battle.
- Cyan Scissorbeak (Variant 2) - These come in 3 colors, Purple, Red & Cyan. Search the upper-left of Eribu, the rare one leaves a stream of light.