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