Ah. I generally just play on the default difficulty, but yeah, I can see how a smarter opponent would recognize that poison+guard is very powerful, especially if the player isn't using Fire.
You can, but not before the game menu becomes available, which doesn't occur until after the tutorial fight. And I think during the tutorial fight, the difficulty will be whatever it was most recently set to (if different from default), even if it was most recently set on a different save.
(Unless it's one of the fights that defaults to always normal? It may be.)