Boss Battle Second Question

How do i make my boss take damage from the projectiles that my alien shoots? Do I have to make it solid?

Make it so that when the boss hits the projectile, he takes damage.

Do this by making it so that he needs to be hit however many times he needs to be hit by the bullet for him to be destroyed. It sounds complicated, or confusing, but there are many examples in peoples games, so check out the logic and behaviors in others games and maybe you can lean some new things (With all do respect).