Here is the full list of all 14 Spear Master achievements.
Defeat Chubby in less than 50 seconds
Defeat Inferno in less than 40 seconds
Defeat Jester in less than 50 seconds
Defeat Plague in less than 60 seconds
Defeat King Tarr in less than 50 seconds
Purchase all items in Wally's shop
Successfully finish the tutorial
Destroy 150 balls
Destroy 500 balls
Destroy 950 balls
Split a ball 4 times in a row without letting it touch the floor
Unlock all the secret areas
Collect 1000 gems
Finish the game with less than 50 deaths