Type in "KRIS" on the option screen and you will then be able to skip through the levels with the 'F1' key.
When you collect the pod that produces four bases, placed at equal intervals along the bottom of the screen, don't just leave them there. If the invaders are quite low, shoot the bases up the screen and into the alien swarms. This way you are guaranteed of killing aliens as they die upon contact with bases, rather than allowing the space fiends to destroy them before they come down.