This is one of the big ones and will likely be one of the last achievements you get in your playthrough. For this achievement, you’ll have to fight every Monster in the game, from the lowly Aptonoth up to the fiercest of Elder Dragons.
Thankfully, Monster Hunter Stories 3: Twisted Reflection has done away with High Rank in favour of the new Ecosystem mechanic, so you won’t have to re-fight the same monster at Low and High Rank.
This is another achievement where I suggest using Monster Buddy to help find which Monsters you have left to find and fight, as there are so many Monsters in this game, it would be impractical to list them all here. They're hard at work adding the latest game to their site, here:
https://monsterbuddy.app/ The Monsterpedia counts Invasive, Exotic (Ogden's Side Story), and Feral Monsters separately, so you'll have to find these too. There are a number of Feral Monsters that appear as quests, such as Feral Deviljho, so make sure you complete them all too.
It may also be worth noting that the Monsterpedia in Monster Hunter Stories 3: Twisted Reflection has an
Endemic Life tab, and these
may contribute to its overall completion percentage.
Once you’ve beaten the last one and your Monsterpedia is 100% complete, the achievement will be yours!
Like many of these achievements, you can check your progress in your Ranger Records by pausing the game, navigating to “Ranger Files” (one tab to the right), and then selecting the Ranger Records dossier (the red one on the right). The
Monster Encyclopedia medal is the second medal on the bottom row.