As above, it is indeed triggered by the amount of bullets you feed into the midboss drones..
I´ve now 1cc´d the game with Ageha (36M) and am attempting to get to 40~ish scores, and here´s how i handle that midboss for now:
1. Laser away at him to a comfortable level. We do not want him to die because of the (possible) lasering at step 4
2. Kakusei when he spews the drones, quickly. You want to be at the right side of the screen so you can hit the two right drones..
3. We are at the right side of the screen. Autofire a few times so the right two get hit and quickly go left to fire at the left two drones as well.
4a. You can now either laser the front line of drones, which is firing the faster bullets or ..
4b. Use tap shot (a) to fire bullets to kill more precise. Then repeat step 3, moving from left, kill, kill, to right, kill kill -> back to step 3
Depending on how much green gems you have it's laser or shot to kill the drones and move on. Laser is easier, you can kill two in one go

If someone has a proper stage 1 ageha 1000 gold replay somewhere on YouTube, i would love to see that as well. Still missing something and ~800 is my best so far, 3.2M. Must do better..