The narwhal is cute. My score was 460.
So far it seems that there is much less need to be aware that the boss's mouth has opened.
The foremost strategy I think is to shoot down UFOs as much as possible and return to the center of the sky when there are no UFOs. This is because if the boss's mouth is open, you can score, and if not, that is still the most flexible position.
If the score you get from the boss's weakness is greater (or other rewards, such as lives or destroying UFOs across the screen), but the position is not necessarily in the center, then you may have a dilemma, such as whether to prioritize shooting down UFOs or attacking the boss.
Or maybe you have a more interesting gimmick in mind.
Whatever it is, this game is fun enough at this point. Please implement music, sound effects, and bosses. I'm looking forward to it.