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