After 4 tense, disappointing battles, I finally gained the upper hand at lvl 11. I was so frustrated by my earlier matches that I left one tiny red bubble alive so I could just enjoy my victory.
It seems like the best strategy is to wait for the computer to overexert its forces on a tiny bubble, then counter attack by taking the larger bubble that just fired... easier said than done of course.
It's hard to tell when it's best to take a smaller bubble or to wait for the computer to weaken a larger gray one... only way I know is through trial and error.
 "Nano War"){kind=icon}
 "Pandemic 2"){kind=icon}