Reminds me Titan Souls in some way, that you need to wait for boss to open to hit his weak spot, which is nice mechanic. But I think there are 2 main issues:
1. Camera. It is very zoomed in and makes a bit hard to dodge asteroids. Perhaps camera could shift a bit towards the cursor? So you could check if there are any asteroids behind you and if it is not, you can attack the boss. Fullscreen could also solve this issue, but actually there is another problem - you have itch.io fullscreen button, but it doesn't work for your game (at least not for me on Chrome), like it makes game fullscreen, but it doesn't actually stretch.
2. Map bounds. You can't see them which makes it a bit hard to navigate. Like boss can stuck at the bottom of the map and it looks like you can go under him to hit his weak spot, but you actually can't and you need to lure it to position above, when again, it is hard to tell if you lured boss enough. So perhaps something like asteroid belt would be nice to see as marker for map boundaries which also could complement flying asteroids. Or well, can just increase map size to like 2x, it should be enough.