Here is the full list of all 15 Hexle achievements.
Complete a daily with no error
Complete a weekly with no error
Complete a hard 8x8 grid with no error
Complete a master 8x8 grid with no error
Complete a master megaboard with no error
Complete 7 consecutive dailies
Complete 4 consecutive weeklies
Reach a metascore of 1000
Reach a metascore of 5000
Reach a metascore of 10000
Reach a metascore of 15000
Reach a metascore of 20000
Unlock all game modes
Unlock all palettes
Uhhh... Oops?