Voici ce que j'ai trouvé sur GameFAQs :
TourianTourist a écrit :Clearing all 50 challenges unlocks the possibility to fight Gramps, whenever you want. He goes under the player name "Legendary", has all 50 medals, the best possibly gear, 20 hearts and two random items. He will chose from the upgraded versions of Ravio's items, as well as blue, yellow or purple potions. The arena will be chosen randomly as well, which includes the otherwise rare desert arena, where he usually picks the Nice Sand Rod as one of his weapons.
To battle him select "challenges" and exit the list. You can battle him as often as you want, but his bounty is only worth 5 Rupees. If you beat him for the first time, there's even a small story as to why his bounty is so low.
Pour les anglophobes, en gros ça dit que quand tu réussis les 50 challenges, tu peux effectivement affronter le papy tant que tu veux, il est très fort et prend les items évolués, ainsi que les potions. L'arène sera choisie aléatoirement et on a + de chances de tomber sur une arène désertique qui apparemment est assez rare. Gagner contre lui rapporte la modique somme de... 5 rubis. (non, je n'ai pas fait de faute de frappe, c'est pas 500 rubis ou 50 rubis, mais bien 5

).
Le reste de l'article peut être trouvé
ici.