Head into the evolved game and realize that the classic game completely blows when compared to this one.
After you’ve done that you’ll have to kill ten saucers. The saucer is the enemy that is shaped like an oval and floats around the area doing nothing. You can go to “Help and Options” at the menu and then see a picture of one if you still don’t understand.
The main trick to killing ten saucers is simply shooting them when they spawn. Every time you kill one the respawn timer for the next is reset, so killing them sooner will cause the next to spawn sooner. Survive long enough and you should be able to pick this one up quickly.