Here is the full list of all 8 Sudoku achievements worth 50 gamerscore. It takes around 1-2 hours to unlock all of the achievements on Windows Phone.
Reach the Ultimate rank.
Give up on a puzzle to earn points.
Finish a puzzle without using any pencil marks.
Solve your first Expert puzzle in Classic mode.
Solve a puzzle in under a minute.
Finish a puzzle in each difficulty in both Classic and Lightning modes.
Max out the combo multiplier in Lightning mode.
Finish a puzzle without using any powerups as rank: Player or higher.