These are some beautiful piano parts! The OST as a whole is very emotive, but I’d maybe go for something a bit more active for the battle theme to make it contrast with the first track.
I think it can be cool to have a somber piece playing when fighting a boss, but that’d depend on the primary emotion you want to communicate – if it’s not fear or danger, but sadness, then in that case it would work, I think.
A very good submission!