Okay, I've uploaded a new build for you to test. I tried your suggestion of bumping up the speed of the bottom sharks by 20% at level 12 and it worked great to allow more sharks to get pass the line of fire forcing the player to move out of the corner. However, it obviously doesn't solve the problem in the levels leading up to level 12. So I got to thinking, what if instead I compensated for the increased player fire rate (which is the root cause of the problem) by adding a small delay before they can fire again. And it looks like that does the trick. Now after level 7 a lot more sharks at the bottom get through the line of fire even if the player is spamming the fire button.
The only question remaining now is at what level to cap the speed of the sharks rising up to attack the player after which they will remain constant. It should probably still be around level 11 or 12, based on the previous build, but I've set it at level 14 in this build, just so you can test out level 13 and 14 if you want to see how it feels. Then again, it might need to be even earlier than level 12. It's your call.
Overall though I'm feeling good about the build. I still can't make it past level 7 without cheating, but on the plus side I'm doing much better in the bonus rounds. Almost certainly because of the faster fire rate, but I'm telling myself its because I'm getting better ;-)
