Start of by selecting any character you want and pick 'Forest Level 5'. Then when in the level aim to get the power-up called 'Red Mushroom' as this will give you increased speed and invulnerability for 15 seconds. Now just keep doing laps around the course picking up crystals (the max you can collect on this level is 800).
If you have a further 2-4 controllers then plug them in and assign a character to them. Due to a glitch any crystals or kills gained by Player 1 will also be gained by the idle characters. So this means that if Player 1 where to get 800 crystals with 3 idle players on the map it will add 3200 crystals to the achievement tracker.