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