Most shmups have you die in only a single hit. However, not many make you invincible, but reset your multiplier whenever you take a hit. This is the concept of
Obsessive Nuclear Testing Disorder, an entry into the
Experimental Gameplay Project's
Unexperimental Shooters theme. It's not the most amazing game ever made, but it's certainly fun and does a good job of making you think about where shmups can go.
Obsessive Nuclear Testing Disorder involves you gunning down ships to build up your multiplier. Every time you reach 7000 points, you get a nuke. You must then use these nukes on waves of enemy ships to get the real, recorded score. It balances reward and risk very well, although once you get so much multiplier, your nukes are self-sustaining.