I personally found this easiest in gravity mode. It worked into my strategy while going for a score of fifty. I started by getting as many orbs as possible sitting at the far end and found that bouncing orbs off the sides of the playing field would take their momentum away so when they were being pulled they would end up near an existing orb and therefore would be smaller, leaving room for more.
Then I made sure that I would have a large orb near the bottom to fire around, this helped to prevent them bouncing back to the death line, and also made them curve around to join existing orbs at the top of the field.
This may take a few tries. I hope this works well for you as it did for me. Happy shooting!