Here is the full list of all 20 Flockers achievements.
Look at what you’ve hurt.
Kill 500 zombie sheep
Get 20 punk sheep moshing together.
Explode 2 sheep while they are in the air at the same time.
Complete a user-created level.
Three star everything
Blow up 999 sheep.
Complete every level (including bonus levels)
Three star every level from the first theme.
Find and unlock all the secret bonus levels
Save 100 sheep from fall damage by landing on something soft.
Three star 2 boss levels
Collect 30 golden fleeces
Try out 10 different styles of sheep (blood included).
Collect 20 golden fleeces.
Unlock a secret level.
Complete the first 3 levels.
Get 20 sheep into the exit at once within 1 second.
Have 20 sheep jump at the same time.
Complete any level with a customised sheep.