I'm not a fan of custom bosses, and I'm not the biggest fan of levels that are solely custom bosses. After a while I managed to beat this boss, and there honestly wasn't too much that was special about it. The Rinkas became annoying after a while, and the speed and height at which the boss flew around at made it a hard target. After using the two mushrooms you had given me, I was left extremely vulnerable throughout the rest of the fight, and I wasn't a big fan of this.
It was okay, but it isn't the best custom boss I've seen. The boss sprite itself had 1x1 pixels in it, and it didn't look all that nice. I'm not sure it you intentionally mean't the boss death effect to show two eyeballs floating in the air (I think you wanted only one eye?), but it didn't fit too well with the boss. I also see you left the sprite sheet of the boss packaged in the level file, this is slightly off-topic, but it's not very professional in my opinion.
I feel as if you could have done a better job at this, such as add a second phase to the boss fight maybe?