Here is the full list of all 14 Mervils: A VR Adventure achievements.
Kill all enemies in Level 1 - "The Outskirts"
Defeat Darthate without losing more than 3 hearts from the Mervil Castle
Complete World 1
Complete the Level 2 - "Mine Cart Run" without taking any damage
Achieve a score of 105 or greater on the darts challenge in Level 6 - "Mervil Waterfalls"
Complete World 2
Complete the Sleigh Ride on Level 8 - "The Mervil Slopes" without taking any damage
Defeat Ramaug without missing any shots with the Ice Spells
Complete World 3
Navigate the open seas on Level 11 - "Pirate Waters" without dying
Complete World 4
Defeat "Balazar the Evil"
Collect all the pages of The Great Mervil Book
Find all hidden areas throughout the game