Now at the boss and I just think it's a bit too annoying, but I usually say that at first with boss fights. But as soon as the second stage kicks in, it feels 'unfair', too many things to watch out for without a real clear pattern.
Might give it a few more tries, but I'm not sure to be honest.
Also feels a bit too random that ... spoiler below ....
... you've been upgrading your gun to just simply lose it for the boss fight for no specific reason .