Best strategy for the later levels is to NEVER take over a neutral bubble. Always take over enemy bubbles - let the enemy waste numbers on the neutral ones. Right off the bat, hit one of their bubbles, then just keep sending number to their weaker ones.
I am absolutely shocked by this game. A (real) new aspect - no experience points. I've played a few MMO's, and this is in the category of ingenius that I'll remember forever. No exp, but you still benefit from long-term playing. Good replay value and a steep skill curve. Very very very amazing!
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