Decent game, but unfortunately it suffers from one fatal flaw. An upgrade game must have decent progression in income. Look at any launch game. You will make 1-2 dollars the first few days, but by the end you are making 1000's or 10,000 times that amount. This game does not do this. So the result is ridiculous grinding to get the upgrades needed to beat the boss. If the income issue was fixed the game would be more enjoyable.
Here is a quick and dirty tip if you need some shields in a middle of a stage: Keep one small solider alive and allow him to shoot you before when clearing out an area. His bullets wont be enough to take down your shields and if you wait there you can fully recharge.
Its so nice that any award you get in pro, you also automatically get in amateur. Thank you so much for not making us grind out songs when our skill level is well beyond the easier version!