The only boss I've seen skipped has been the Spore Spawn in the speed run on the site (sorry I forget the runner responsible for it, however, you blew my mind with your tactics) Since they're all in the way for essential (at least I think) upgrades needed to go on
Aptly titled PB #66. It is located in one of the rooms you have to drop through the quicksand to get to. It is on the right of the screen, in a little alcove that has an entrance the size of the morph ball. The entrance is opposite a wall and is in a wall itself... about 7 tiles up (guessing) you have to wall jump off the two walls, underwater, without the gravity suit, and time a mid-air morph just right to get into the hole. The worse part about it, is that you can see it sitting ther... laughing at you!!! GAH! If you watch Red Scarlets run... you can see her try for it, but not get it.