(Disclaimer: there's literally only one implemented female character to inflate, and she's the hardest of the three bosses. Easy fight, though.)

As a fetish game, this game's pretty darn cool, the beat-em-up stage before each boss fight is basically just an afterthought, but the bosses themselves are great.
Basically, in each boss fight, you can choose to go for precision, or bullet hell. If you shoot a boss in the mouth when it's open, it'll heal them. If you OVER heal them, they'll start to inflate until they eventually burst at like 4x their max health, but the fight's only challenge is getting them when the mouth is open AND hitting the mouth.
If you fight the boss like any other shmup, the fight gets harder, but you can hit anywhere you want (minus the open mouth.)

The cool bit is that there's an extra big mode for each boss if you beat them the inflation route, the female character nearly goes spherical and loses her top completely.