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