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