The player has two resources: lives and bombs. If the player has no more bombs to blow up your attack with, they’re hit and lose a life. The player had 3 bombs for every life, and 5 extra lives. That means you need to hit them 24 times in total (not counting shield hits) to win the game.
I guess it could have used some more explanation. I’ll probably work on including a full tutorial in the next version.