Don't rush. Buy the Rewinder, and upgrade it always as soon, as possible. With its help - and with good reflexes, of course - you can destroy the
waves before they reach the end ball, and at the end of the stage you can collect more parts, then normally, because the end ball will be much closer to the start point. With a little practise, you can collect 100 parts/level easily. Just don't be too selfish, because it's easy to ruin the whole thing with bad aiming.