Arrow keys to move. If your ammo runs out you lose. Kill all the red monsters. You can hit more than one monster at once with a single bullet. Choose an option (easy, medium or hard) by holding your mouse down and dragging it over the option you want. However, you cannot take more than 5 secs doing this. Easy gives you 15 ammo, medium 10, and hard 5. The one going round in circles can kill you by touching you. Shoot it to kill it. Your ship will follow your mouse pointer's x axis.
Thanks a bunch to jsy6 for the idea, but I didn't copy any of the script. There are still a few bugs I'm trying to fix at the moment.