Looked into a few things
The alien/dragon that gives you the extra ball I mentioned earlier according to gamefaqs
"Get to the bonus round with skulls and green aliens. Kill everything except for one skull. Keep going until a pteradactyl appears. Hit the pteradactyl with the ball to get a million points and a bonus ball once the stage is cleared."
Not the easiest bonus round though.. probably the hardest of the four, those center bumper squares just want to bounce you super fast down the drain and its hard to keep track of the ball with all the enemies at first.
"Achieve a score of 999,999,900 points and the ending will start."
Thats crazy it would probably take like 40 hours to beat.