This one is easier in the middle stages as the beginning stages don't always provide enough coins. Coins are produced from triggering big groups - 5 (4 + 1 you shoot) or more colors and use of the scorpion power-up during the level. Completion of the level with the scarab as far back towards the beginning as possible will produce additional coins. When you have completed the level the scorpion races around the track releasing jewelery and coins. I think the most I have got from this was 6 coins when I blasted the chain back to the beginning and finished it off from its entry point.