My first game was a space shooter where the enemies didn't move (they were animated, though). They just sat there. When you shot at them, sometimes they'd shoot back. They'd take a number of hits to kill.
There was the UFO. One shot, it dies, and shoots a bullet straight down. There was the enemy ship. Three shots, it dies. Each time it's shot, it would make a down-bullet. When it dies, it would make a seek bullet (a seek bullet is a bullet that only moves in your direction when it's first created. It continues to move in a straight line, making it not too hard to kill). There was the BOS (not your normal bos idea. They appeared much more frequently, and were normally reinforced with ships and ufo's). It was huge, and took what, 20 shots to kill. Each time you shot it, it would produce 2 seeker bullets. When you killed it, it would produce a bomb. There was the bomb producer, that every second produced a bomb. And the cluster bomb (or just bomb), which is a bit larger than a bullet, that fell straight down. When it reached 3/4ths the way down the screen, it "exploded". It was indestructable. When it explodes, it produces 1 seeker bullet from itself, 1 seeker bullet from each corner of the screen, and 1 seeker bullet from each side of the screen (the centers). It wasn't too hard to dodge them, though. It was just when you started going trigger happy with the bos, or when there were a few bomb producers on the screen that you tended to get hit.
The scoring worked as follows. Every level you got 30 points. Everytime you got hit by a bullet, you lost a point. After 10 hits, you lost a life (of which you had three). Every 3 levels you gained a life. You only got a highscore I believe if you beat the 10 levels (sometimes I'd put in several bosses. In fact, one of them I think I had 10 bosses).