The proloque level was easy! You just have to get to the checkpoint, where the ship goes through the stone thing....and then it's a peice of cake! You just have to survive. If the ship dies, you will be thrown back to checkpoint, but you didn't die.
That's how I did it.
And I'm searching for a good strategy to beat the 25 waves of skirmish. Any ideas? I know, that you can start on wave 19, but I don't know how much of a help this is, considering that the enemies are much stronger, too..