Here is the full list of all 26 Letters & Legends achievements.
Defeat the Mega Demon.
Do 2500 points of damage to your enemies.
Buy or unlock at least 10 special items.
Create 5 words with 10 or more letters.
Unlock Chapter 2.
Unlock Chapter 3.
Unlock Chapter 4.
Unlock Chapter 5.
Use 30 Flaming tiles.
Earn 30 extra hits.
Find 10 Killer Words.
Level up all 3 characters at least once.
Win any Mega Challenge.
Create 10 words of 9 or more letters.
Create 20 words of 8 or more letters.
Spend more than 5000 gold coins.
Finish a level with only a quarter of a heart remaining.
Use 30 Healing tiles.
Deal at least 20 damage with a single word.
Drink 30 potions.
Finish a timed level with more than half your time remaining.
Create 2 words with 12 or more letters.
Create 200 words.
Defeat at least 10 enemies in a single Endless level.
Shuffle the letters at least 10 times, not counting Anagram levels.
Use every letter at least once.