Here is the full list of all 26 Zombie Ballz achievements.
Killed 50 Enemies
Freeze Camera for 5 accumulated minutes
Defeated Skeleton King Boss
Unlocked Slime Blob Weapon
Killed 25 Enemies
Collect 25 Coins
Unselect all but one earned weapon
Unlocked Meteor Weapon
Unlocked Twisted Roots Weapon
Unlocked Wooden Ball Weapon
Unlocked Ball of Rope Weapon
Collect 50 Coins
Collect 75 Coins
Collect 100 Coins
Collect All Coins
Killed 100 Enemies
Killed 150 Enemies
Killed 250 Enemies
Defeated Zombie Brute Boss
Defeated Zombie Lord Boss
Defeated Necromancer Boss
Saved a Custom Level
Unlocked Whirling Axe Weapon
Unlocked Buzz Saw Weapon
Unlocked Fireball Weapon
Speed up time for 5 accumulated minutes