Analysis of the bosses and how to defeat them. Spoilers ahead.
This guide will tell you what are the bosses attacks, how to avoid them and beat the boss without even getting hit.
They are pretty easy to beat, but this guide might help you get used to these fights and make them more easy. They all work the same way with some minor differences.
First, the boss will just warp around you. (Sometimes, when she does, Kurona also uses decoys that can fool your Pheromone Goggles). When the boss warps, you'll see big particles popping. Always follow the yellow one if it's Kurona, or the pink one if not. (It's usually the particle that has a diffferent color from the others).
When the boss appears, shoot at her like a crazy until her projectiles appeared. Then, stop immediately and shoot those projectiles. (Note: The very first time, she shots them in front of her, so blast her at will, it will destroy her projectiles as well).
Kurona: After some shots, she will pop into around 20 of mini-kuronas. Your Demon Sweeper will reload fast automatically. Stay stationnary and suck up the mini-kuronas passing by. They will be like englued to the vortex and get caught... Once there's around just 6 or 7 of them, move around to get the rest of them. You can also shoot them to go faster. The next phase begins when they're all sucked out, killed (?) or after some time when the remaining ones disappear.
Chiru and Nanako: Instead of mini-Kuronas, Chiru/Nanako will use smoke. Blow it around you to make it disappear and find Chiru/Nanako. Once you found her, keep sucking the smoke around her until she starts moving at you. Then, shoot her like crazy.
The boss will spawn projectiles around you. Start shooting the ones at sight. But when there's red arrows appearing, stop immediately and turn to them to shoot at the projectiles coming at you. Once they're all gone, the boss will charge at you. Shoot the HELL outta her.
Then, the boss warps again and it all restarts to the 1st phase. Do these phases two more times.
After that, (except if it's your first fight with Kurona) the boss will either pop out your Demon Sweeper or be in a "finish her!" state. Activate the Demon Sweeper in front of her until her clothes comes off. And voilà.
Note that, when you'll max out your sweeper and goggles, this fight will be so easy, the boss might not even have time to shoot projectiles at you at the beginning! So, boost yourself up asap ;)