Actully Ganon's version is better. My version is just a cheap Final Form version that doesn't involve a body.
So I read your suggestion AEM. The not invinisible, still suffers damage, and barely suffers knockback is already my idea that I was planning to add to Ganon's idea (I just haven't said this yet). It would be pretty cool to have a force field containing Toon Zelda like the many Zelda games have during a final boss battle while fighting as a Phantom, but the Ganon's idea forces players to use defense to this FS propery. Phantom Zelda will not just charge at her oppenents, but stay close to her body in order to defend it. This would just be like Spirit Tracks, Phantom Zelda will help and defend Link (Toon Zelda's body) in puzzles and defeat enemies and stick close to him (if, of course, Link doesn't command Zelda to go far). Almost as good as Four Swords, this relies on teamwork, and I want Phantom Zelda to defend her body like how Link and Princess Zelda would defend each other (or how Green Link defends Blue, Red, and Purple).
Besides, you never stated that their was a time limit, so players would obviously use Spirit Zelda to evade attacks for a long tim before the match ends. Also, she reqiures Byrne, a major enemy, to activate the force field for her. I rather not have her rely on others, ecspecully enemies, to do something for her.
My plan was to have her immune to weak attacks and projectiles and not flinch at all (tilts, Bowser's fire breath, Pit's Angel Ring, Link's arrows), stops in her tracks (or slows down) by medium attacks, aerials, and medium strength projectiles (Falco's reflector, Link's bombs, Link's boomarangs), and get really low knockback and fall down (as if a Phantom attacked her) by really strong projectiles and attacks (Smash attacks, Ike's Eruption, Samus's fully charged charge shot, and Link's spin attack).
Understand?