well, if link turned oni link ( fierce diety ) then he could swing his sword and shoot out a beam of energy like he did to majora, or in the old zelda games when you had full health. mario could have a giant mushroom like in new super mario bros and turn so giant that he runs around squishing people, leaving them to be kicked to kingdom come. samus could unleash a barrage of phazon-based attacks and stun people because of the toxicness.
link could also get out the stone mask (or was it giant mask) whichever one you got at the stone temple in majoras mask, then smash the ptoodie out of people, but be invincable.
just my thoughts.

Fierce Diety...(love the emphasis)...I never knew that eating became an issue in LoZ. AFAIK, the only things that Link takes are Milk, Potions, Soup and Bee Larvae. But other than that diet, I didn't think you'd need another diet to become fierce. All I know is you just need a mask. And why would you want a diet if you could become a god (deity).
Yes, it is the Giant Mask...the stone mask made you inconspicuous, unseen and undetected...except by some smart gerudo pirates.
Invincable - You know this word just amuses me. Reminds me of how I used to spell the word irresistible like this: irresistable. But this takes the cake. I never heard this word being said: in-vin-ka-bul or in-vin-kay-bul or in-vin-sa-bul. Maybe just maybe, you're pertaining to an invincible cable...but honestly, I would have made it inviscable as cables couldn't be invincible (but maybe against wear and tear...) but more of invinsible where they use it for special effects like flying. Maybe there are inviscables in SSB64 because they are dolls anyway held by the Master Hand...so sure.
Sorry if you have become the butt of spelling jokes but I have to admit you make funny mistakes.
scale-wise i actually thought that a giant super mario would be smaller than gigabowser, if not the same size.. but if your talking moveset-wise, your right
I was thinking the mega mushroom is enough. Speaking of which, do you think giant versions of the characters could take on giga bowser? I see it more probable that way.