I managed to do it with Captain Falcon, but it took me several weeks of trying to pull it off. I don't know that I could offer any advice...except to get really good at "dodging" with the block button if you want to hope to pull it off. If you don't know about dodging, it's when you hit down on the analog stick (or whatever duck is on your controller setup) while holding the dodge button. This can be done on the ground or in mid-jump and it's an excellent way for projectiles or even entire bosses to pass through you without damaging you. Also, I studied a couple YouTube videos of Boss Battles that people had uploaded. It made me see new ways to fight some of the bosses.