I love the epic orchestration and the spooky SFX are a nice addition.
I would say that although I like the thunder sounds, they could be less frequent as sometimes it feels like it's competing with the other instruments for attention. I liked the bit where the music dropped out at the end to let the SFX have a spot to shine.
I also really like the melody. It fit with the boss fight theme and kind of reminded me of something I'd hear in Zelda.
The piano in the background was also a nice addition.
Awesome work!