MONSTER HUNTER: WORLD

This achievement obviously requires the Iceborne expansion. Gaining research levels in Iceborne works the same way as in the base game, you increase levels by examining tracks, observing turf wars, capturing and killing monsters. Note that in Iceborne, any monsters that were not in the base game have a maximum research level of 4, while monsters that were first available in low rank now have a maximum research level of 9, and monsters first available in high rank now have a maximum research level of 7 (with a few exceptions that you'll see below). Here is a list of what my research levels were when the achievement popped for me:

Zorah Magdaros - Level 4
Great Jagras - Level 9
Kulu-Ya-Ku - Level 9
Pukei-Pukei - Level 9
Barroth - Level 9
Jyuratodus - Level 9
Tobi-Kadachi - Level 9
Anjanath - Level 9
Tzitzi-Ya-Ku - Level 9
Paolumu - Level 9
Radobaan - Level 9
Rathian - Level 9
Great Girros - Level 9
Legiana - Level 9
Odogaron - Level 9
Rathalos - Level 9
Diablos - Level 9
Kirin - Level 9
Dodogama - Level 7
Pink Rathian - Level 7
Azure Rathalos - Level 7
Black Diablos - Level 7
Lavasioth - Level 7
Uragaan - Level 7
Bazelgeuse - Level 4
Deviljho - Level 4
Nergigante - Level 4
Kushala Daora - Level 7
Teostra - Level 7
Lunastra - Level 7
Vaal Hazak - Level 4
Xeno'jiiva - Level 4
Beotodus - Level 4
Banbaro - Level 4
Viper Tobi-Kadachi - Level 4
Nightshade Paolumu - Level 4
Coral Pukei-Pukei - Level 4
Barioth - Level 4
Nargacuga - Level 4
Glavenus - Level 4
Tigrex - Level 4
Brachydios - Level 4
Shrieking Legiana - Level 4
Fulgur Anjanath - Level 4
Acidic Glavenus - Level 4
Ebony Odogaron - Level 4
Velkhana - Level 4
Seething Bazelgeuse - Level 4
Blackveil Vaal Hazak - Level 4
Namielle - Level 4
Savage Deviljho - Level 4
Ruiner Nergigante - Level 4
Shara Ishvalda - Level 4
Zinogre - Level 4
Yian Garuga - Level 4
Brute Tigrex - Level 4
Gold Rathian - Level 4
Silver Rathalos - Level 4
Rajang - Level 4
Stygian Zinogre - Level 4
Frostfang Barioth - Level 4

I have read online that Rajang and Stygian Zinogre are not required for the achievement, so your mileage may vary on those two. Here is a list of monsters that I did NOT have maxed out when I unlocked the achievement:

Kulve Taroth
Safi'jiiva
Furious Rajang
Raging Brachydios
Behemoth
Leshen
Ancient Leshen
Alatreon
Fatalis

Good luck and happy hunting!