Here is the full list of all 43 Starlight X-2: Space Sudoku achievements.
Complete a level 6x6 in less than 1:20
Complete a level 7x7 in less than 1:45
Complete a level 8x8 in less than 3:15
Complete a level 9x9 in less than 3:45
Complete a level 10x10 in less than 4:30
Complete a level 11x11 in less than 6:00
Complete a level 12x12 in less than 7:15
Complete a level 13x13 in less than 8:45
Complete 25 levels
Complete 50 levels
Complete 100 levels
Complete 500 levels
Complete 1000 levels
Complete 5000 levels
Complete 25 6x6 levels
Complete 25 7x7 levels
Complete 25 8x8 levels
Complete 25 9x9 levels
Complete 25 10x10 levels
Complete 25 11x11 levels
Complete 25 12x12 levels
Complete 25 13x13 levels
Complete a hard 5x5 level without using any help
Complete a hard 6x6 level without using any help
Complete a hard 7x7 level without using any help
Complete a hard 8x8 level without using any help
Complete a hard 9x9 level without using any help
Complete a hard 10x10 level without using any help
Complete a hard 11x11 level without using any help
Complete a hard 12x12 level without using any help
Complete a hard 13x13 level without using any help
Play for 5 hours
Play for 10 hours
Play for 24 hours
Complete the Pack: Earth
Complete a Daily Pack
Complete at least one daily level for 3 days
Complete at least one daily level for 7 days
Complete at least one daily level for 10 days
Switch off the light
Use 10 hints
Use 20 hints
Use 50 hints