Whenever you can aim to upgrade your shooting speed and power. With each upgrade power will cause your bullets to get more damage power and speed will allow you to fire your gun more quickly.
Keep in mind that with each boss fight all of your enemies will be much stronger so it is essential to upgrade.
Dark blocks of gold are worth five and yellow ones are worth two per piece.
To earn free gold in this game go to the shop menu and you will be able to earn 500 gold instantly for liking the game on Facebook, and 500 more gold for following the game on Twitter.
Complete missions in order to earn more free gold as well.
To get two free diamonds go to the main menu and head to the theater to watch advertisements for gems. You can watch as many as you want.