Well, you have to collect the blue 'pods'. There are ten per level and a counter in the corner of the screen.
As for the aiming, the reason you have to aim for the robot's feet is that the laser is supposed to be coming straight down from space.
The spawn points can't be destroyed or it would probably be too easy. I did think about putting in walls, but because the laser is aimed independent of your position you would effectively be able to shoot 'through' them. You could just hide and pick them all off.
But glad you liked it. Simple low res art is about all I can manage.