Yea I felt like you have to use your sword on them, but that didn't work for me, so I don't know if it was bugged or maybe I was just missing it. It was hard to test with how little of them spawned. Also it was hard to see when a projectile spawns when fighting enemies, so maybe having some type of sound that it's coming would help.
There actually is a sound! It's honestly just really quiet though. But yeah, the boss projectile spawning was a bit bugged, unfortunately. Whenever I'd re-export the model for it from Blender, it'd wipe my animation data in Godot, and that was where the projectile spawning logic was being called from. If I had more time I deeeefinitely would have improved this aspect of the game though, very thankful for your feedback!