The Crusher mini-game is played between each mine in the main game and each mine in weeklong events. 

In the bonus stage of this mini-game, you control a Crusher Truck. Your main objective here is to smash rocks and subsequently collect the rewards they drop. The timeframe given for this task is just 30 seconds, meaning you'll need to maximize your efficiency during this short window.

The prizes you stand to gain vary from Elixir, Gems, and Cards to Boosts. Each level in this mini-game is systematically balanced to contain increasing rewards as you proceed further into the game.

A useful tip to remember is about the Epic Cards. Levelling up some of these cards will enable upgrades to your crusher truck. These upgrades will significantly aid in collecting even more rewards. 

Good luck!