Yami, at the end of the game. He has five different forms, the last being his true form. Yami literally means darkness in Japanese. Yami will steal all of Ammy's brush powers and will slowly give them back when fought.
Ozma is the hardest boss in the game
Beat the final boss, Yami, in the Ark of Yamato.
It matters what game you are playing. For example i play Wizard101 and so far the hardest boss to me is Jabberwock.
orcram
The hardest boss is Sigma
the dog
you
The Moon Lord
The next hardest boss I'd say is Kolokotos.
Megahammer in world 3 ( Finel Boss in world 3)
the hardest boss in kingdom hearts recoded would either be roxas or sora's heartless in it's last form
Nemesis