Here is the full list of all 52 Letter Quest: Grimm's Journey achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Earn 40 crystal stars
Earn 120 stars
Spell 200 double-letter words (ex: 'bubble')
Purchase 50 merchant items in battle
Spell 50 six-letter words
Complete 50 quests
Defeat 250 monsters
Swap all letters 50 times
Earn 99 stars
Use 2500 vowels
Bust apart 100 gem piles.
Purchase 5 books
Use 5000 letters
Purchase 30 upgrades
Earn 20 crystal stars
Open a treasure chest with no incorrect guesses
Reach max level with one book
Spend 25000 gems
Take 10000 damage
Spell 3 words with no vowels
Earn 1000 book XP
Use 100 crystals
Deal 25000 damage
Open 10 treasure chests
Earn 15000 gems
Use 500 corner letters
Spell the word BACON
Spell 10 seven-letter words
Spell a word that contains two 'Y's
Purchase 10 potions
Spell 5 eight-letter words
Use an entire row in one word 3 times
Earn 29 stars
Finish a stage with full health
Deal 10000 damage
Use 500 letters from the top row
Use 25 plague tiles
Spell a word containing two or more crystals
Leave a monster with only one health
Spell a word that does zero damage
Beat a stage with only 1 health remaining
Spell 2000 words
Spell 3000 words
Spell 25 palindromes
Fill the entire board with plague tiles
Fill the entire board with duplicator tiles
Defeat 5 monsters with exact damage
Dodge 50 attacks
Do 100 critical attacks
Lose from taking tile damage
Fail to open 20 treasure chests
Spell the word BANDITS