This achievement is for "smashing" your ball 20 times. *Insert joke about smashing balls here*
Anyways, the smash mechanic is introduced to you in level 30. You basically use a drag block to pin your ball against a wall to allow it to free fall straight down. Doing so stops it's side to side movement, avoiding death.
The counter on this seems a bit off as I did more than 20 before it unlocked, so don't worry if you feel it should have unlocked sooner.