This achievement can be completed almost immediately, with nearly no prep work required. All you need to do is gather 40 pink slimes, as they're the most plentiful, and throw them in a corral. Be sure to buy the high walls upgrade so they don't escape. Afterwards, simply jump in with them!
Here's my video for every video if you'd prefer. Skip to 1:34 for Ball Pit.
Happy Hunting!