Here is what I do for a good score:
For the first couple of shots shooting in up direction is ok, after that switch to shooting more in the down direction and to either side, as you don't want to hit the human and miss them falling down. Human comes from top. Once you see 1 point for shooting planes, usually after getting a couple of humans, then search out boss on the level immediately. I usually go right to find boss and try to get 1 more human on the way. I suspect folks with high scorer than mine get 2 humans on the way.
I attack all bosses from right going left, same direction as the boss. If you find boss to the right, shoot boss as you are flying towards him, then go up and you find another boss. It's the same boss and finish him off. I usually shoot from bottom right of boss, and angle shot or level shot to kill boss.
If you shoot human and miss them, don't bother to continue, just run your plane into another plane/bullet until all lives are lost. I wish rom has press * # to reset like most Jaguar games, but its missing that. Same thing if you hit something with your plane, just get rid of the rest of your lives.