this maybe the easiest challenge... but maybe it's tough, too!
well, the callenges are:
-. Defeat Roxas with Kingdom Key keyblade, and only uses Dodge Roll as the Growth ability...
=. Defeat Sephiroth with Kingdom Key, no growth abilities allowed
for more advanced challenge: no Second Chance and Once More abilities equipped. (Exclusive Seph. Part: no MP Haste allowed)