PURE WIN
I haven't played a game this fun and addicting in a long time. I love the way you can buy upgrades mid-battle, without going through waves or levels. It's really great.
The trick I figured out is to start off attacking, and keep attacking continuously without relenting for a second. that way the enemies concentrate on you and not on your space station. First buy the weapon upgrade, then the shield upgrade. If you play enough, buy the smart missles next. They REALLY help in killing enemies because you don't have to aim for the missiles to hit. Basically, keep upgrading weapons until you destroy the outer, slower-moving orbs (you have to attack continuously to kill them because they regenerate health when not being attacked), then buy an engine upgrade or two. For the rest of the game, concentrate on getting your weapon upgrades, then your shield upgrades, then the special upgrades, which I found to be extremely useful.
Never forget: Attack, attack, attack! Don't tryo to defend, because for one thing that's not the object, and second you'll lose that way. Attack the enemy satellite/protection orbs and the enemy attack orbs continuously, and winning is a breeze.