Play as Metal Mario:
Beat the story mode on normal or higher difficulty mode using Wario, and without losing even once, and you will unlock Metal Mario.

Play as Vampire Wario:
Beat the story mode on normal or higher difficulty and you'll unlock Vampire Wario.

Make Computer More Challenging in VS Mode:
Select your computer opponent and hold L and Press A to increase the difficulty.

Make Computer More Challenging in Story Mode:
Highlight Hard Mode and Press Z to make it even harder.