Here is the full list of all 21 Whipseey and the Lost Atlas achievements worth 1,000 gamerscore.
Drowned an enemy.
Used the whip to swing.
Cleared the Beach Level.
Defeated 50 enemies.
Clear the Forest Level.
Clear the Desert Level.
Clear the Snow Level.
Clear the Castle Level.
Hover for at least 3 seconds.
Clear a level defeating all enemies.
Ride a giant enemy from the Snow Level.
Clear any level without using a potion.
Clear any level without taking damage.
Clear any level without dying.
Clear a boss without taking damage.
Clear all bosses without taking damage.
Clear the beach level boss without dying.
Clear the Forest Level boss without dying.
Clear the Desert Level boss without dying.
Clear the Snow Level boss without dying.
Clear the Castle Level boss without dying.