Here is the full list of all 8 Shmadow achievements.
Die a total of 100 times
Achieve 100,000 points on Nightmare
Achieve 400,000 points
Get 100,000 points in under 100 seconds
Kill over 9000 enemies
Defeat all the bosses
Defeat the same boss twice
Be fully charged and have a power up and don't use either to defeat a boss