I discovered that the poison and shield is crazily broken for ai cuz I'm playing in expert mode so I turn to easy mode lol
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.)