Here is the full list of all 12 Castle of Pixel Skulls DX achievements worth 1,000 gamerscore.
Finish Level 20
Finish Level 15
Finish Level 10
Die 20 times
Die 10 times
Die 5 times
Finish Level 5
Jump 40 times
Finish Level 1 without dying
Finish Level 1
Jump 25 times
Jump 10 times