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