I'd be interested to know too. A couple of slim possibilities come to mind. You can leech points on many of the bosses and some mid round bosses by only destroying their obstacles. I can't see how you can get the extra 350k from any of these though.(posted 8704 days ago)In level 1 you can just destroying the little monkeys the boss throws out until the time runs out - 100 pts per monkey.
On level 2 you can keep blasting the eyeballs the Snowman produces - 50 pts per monkey so not as good as level 1.
On level 5 you get 50 pts every time you hit the feet or hands of the end round boss. This can be done very quickly - as fast as you can shoot.
None of these seem a lot faster though than jumping the green rings at 100 pts per bounce though as you mentioned.
Cheers, Tim