You must destroy 25 crystals in "Crystal Rush" mode. To unlock this achievement quickly, start a game with these settings (requires 2 controllers):
- Difficulty Bots: Disabled
- Game mode: Crystal Rush
- Crystal life: 5
- Level: Ruby vs Sapphire
Each game will destroy 7 crystals. The achievement will pop at the end of the 4th part. You would unlock Spike Knight at the same time.