Difficulty spike was large, and was way too hard for me with the second wave of attacks. 

I like the bullet hell vibe, however the second phase is a really (too) tough and I can’t pass it. Maybe, it’s a problem on my side, but game opens in small window which can be larger, but it the stage is still too small

Looks good ! It's really hard though, especially once the second pattern starts. I didn't get if I was damaging the boss or not ?
The introduction is also nicely done.