Here is the full list of all 18 Gravity Panda achievements.
Pass Level 6 in Chapter 1
Pass Level 9 in Chapter 1
Get the star in Level 11 in Chapter 1
Pass Level 2 in Chapter 2
Pass Level 6 in Chapter 2
Get the star in Level 10 in Chapter 2
Pass Level 2 in Chapter 3
Pass Level 8 in Chapter 3
Get the star in Level 10 in Chapter 3
Pass Level 3 in Chapter 4
Pass Level 8 in Chapter 4
Get the star in Level 10 in Chapter 4
Pass Level 1 in Chapter 5
Pass Level 9 in Chapter 5
Get the star in Level 10 in Chapter 5
Pass Level 4 in Chapter 6
Pass Level 7 in Chapter 6
Get the star in Level 10 in Chapter 6