The aim is simply to kill all 20 ships. The AI is fully implemented but not properly balanced yet and they sometimes shoot in mysterious directions. However they do usually target your direction and despite having no swarm mechanics do tend to act in a swarm-like behaviour.
I've mostly been playing against 50 ships when doing testing and that can prove quite difficult. You shouldn't reach 50 ships for a long while in the main game though.
Tips are to just keep moving as much as possible, but not TOO fast or you'll outrun your bullets and run the risk of smashing into ships. If you die (energy bar is at the bottom) the demo will exit. You only get one life. ;o)
Currently things go a bit weird at the edges of the level. For one the AI will get confused and start flying in the opposite direction, ships on the opposite side won't be drawn and bullets don't have "move to correct side" parts attached. So, if you see a big red line, I suggest flying away from it, although nothing will go spectacularly wrong if you don't.
In other news, I did a fair bit of yesterday and today's development under Linux, where the game just runs. I'll write a post about my reasons for choosing C# and OpenTK (this being a main one) at some point. For now, you can see what it looks like here, ignore the tearing, that's just a vsync issue. I'd not quite got the radar implemented there, so no red spots.
I tried to take a video of the tech demo earlier but ran into some issues unfortunately. I'll try again later though.