Here is the full list of all 23 Otteretto achievements.
Classic mode: Reach 2000 points.
Classic mode: Make a palindrome using all 5 colors.
Classic mode: Reach 1000 points.
Classic mode: Clear the first 5 levels.
Classic mode: Clear the first level on your first move.
Classic mode: Make a palindrome of length 18 or longer.
Classic mode: Reach 2000 points without using Bonus Rows.
Classic mode: Reach 3000 points.
Solve 4 easy puzzles.
Solve 3 medium puzzles.
Solve 2 difficult puzzles.
Solve a nightmarish puzzle.
Containment mode: Clear the first level.
Containment mode: Clear the second level.
Containment mode: Clear the third level.
Containment mode: Clear the fourth level.
Walls mode: Clear the first level.
Walls mode: Clear the first 4 levels.
Walls mode: Reach 1000 points.
Walls mode: Reach 2000 points.
Spell O T T E R E T T O.
Complete all 20 regular achievements.
Reach 5000 points.