This is the 50k bonus on stage 1:
Star Force Stage 1 + 50k bonus
Some pointers:
When the music changes (at 0:50) no enemies will appear and you know a mid-boss is coming up. Use this cue to position yourself near the top centre of the screen
The boss flashes white just before the two side pieces enter the screen.
Best way to judge this is look at the distance between the bottom piece and the boss. Get familiar with how close it will be.
You need to hit approximately 8 shots from when it flashes.
Any shots you hit BEFORE this, it will add to the total needed.
Playing the vid back at 0.25x speed, you can see I shoot a couple before it flashes and manage to get the bonus - anticipate it, being early shouldn't cost you.
Not much more I can say to help. I save-stated this until I got it consistently.
Good luck! Hope this helped a little

---------- Post added at 12:12 AM ---------- Previous post was at 12:06 AM ----------
Also for the end of stage boss:
If the hundreds digit in your score is EVEN he will go to the RIGHT
If it is ODD, he will go LEFT
