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