Here is the full list of all 25 Calcium Contract achievements.
Kill 1000 enemies
Kill 7 enemies with a single attack
Kill 3 roosters with a single scythe swing
Catch all treasures in every level
Destroy all clocks in every level
Finish the game
Catch a treasure
Kill a treasure rooster with a scythe just before it disappears
Cause an enemy to fall to their death
Finish the Docks
Finish the Industrial District in under 90 seconds in one go
Finish the Caves in under 3 minutes in one go
Leave no cat alive in the Cats Temple
Don't kill any cats in the Cats Temple
Freeze 7 enemies at the same time
Kill an enemy with their own projectile
Finish the Mountain Halls without picking any yeller skulls
Get a little too eager when chasing the Serpent
Get killed by an exploding corpse
Kill an invisible jester
Cause a berserker to kill another enemy
Get both the armor and the increased damage power-ups at the same time
Deflect 7 enemy projectiles with a single scythe swing
Let the Serpent spread as much as possible
Kill a flying sniper just before they take their shot