This is so much more addicting than i thought it'd be. Surprisingly a fun game. Cleared the first spell of each on easy and the second one on each on normal. Kogasa's first spell is really hard because it's all randomness. Not much else to say about it. Her second spell though is really really fun. I figured out the best strategy to beat it is to stay all the way to the left so the bullet spread is the biggest while not having to worry about horizontal movement at all. Spent the longest on trying to beat it on normal. Mystia's first is also really hard even on easy. I like how the aimed blue shots seemingly end at your X location. Giving some benefit to constantly go closer to Mystia to bait out a shorter blue attack and back away from her for safety. Mystia's second however is my favourite spell in here. It perfectly mixes Flappy bird with touhou and is the most fun one to do since flappy bird was made in design with pillars after all. It was also the easiest to beat on normal.
Hope you add more levels to this. There should also be some more sound effects to make the gameplay a bit more impactful. Especially a sound effect for winning (maybe slow motion before the boss dies?) as right now it's pretty anticlimactic. The boss hitbox should also be a bit higher as it's fairly difficult to stay in the middle for all your shots to land on it. But this game is really a hidden gem despite how it looks.