Thank you very much for the great feedback! The lack of damage feedback on the boss (and regular enemies too) was still on the to-do list when the time ran out. It's going to be in there for the 1.1 version, after the jam rating period is over.
What do you think, flashing when they take damage and a healthbar for the boss?
Also, if we overshot the quality of the GameBoy, we'll just take it as a compliment! XD
I remember being quite unsure about limiting the visual fidelity more to get a more authentic feel or to do everything the jam rules allowed for. It's a tough one, really. On the original GameBoy, even if it was nominally running at 60fps, had most games run quite a bit slower. And things like dynamic particle effects and accurate physics were really a no show. Instead everything would have been done with sprite sheets and a bit of in engine animations. Flexible Frosty is more on the original side of things in this regard (https://itch.io/jam/gbjam-8/rate/751190).