I did it on the first stage as well as "Return of the rattlesnake Jack" stage.
Everything being equal (20x multiplier and fully upgraded charge attack and charged attack range), I preferred the later stage as there are more enemies and the worm is more powerful and split out more small worms at a faster pace.
The disadvantage is of cos the explosive nature of the small worms (tendency to get injured when using your charged attack). However if both your charge speed and range are fully upgraded, then it's not really an issue.
Got it within 30 minutes.
Last edited by banej; 09-28-2012 at 06:11 AM.