That boss's charge attack is surprisingly tricky. It crippled my first run into the boss fight to the point where it felt pointless to continue. I can see why they died to it on stream. Not sure what makes it so hard, I think the wind-up is just kind of long and if you start moving too early you're up against the edge of the screen with not enough time to change direction. Maybe?
Also, the stage is completely impossible without holding formation and trivial if you do. I didn't realize shift did anything at first because there is no feedback from pressing it at the start of the game when you only have 1 Reimu, so I did a few tries without shift before I stumbled upon it.
There probably is a way to make the enemies be not impossibly tanky by limiting the damage they can take per seconds, rather than giving them so much health that a single Reimu can't take them out even if she shoots at them constantly. This would make more formations viable and recovery possible.
Overall fun gimmick game. Could have been slightly more than a gimmick if it kept track of score or something, but probably not much more.