Good IV's and nature aren't absolutely necessary, but they'll help a lot. I would suggest EV training as much as possible, starting around Radomus, I had trouble until I went back and EV trained all my poke's, even with my pokemon near the level cap.
IIRC, I heard that the later gym leaders all have perfect IV pokemon, and I believe that starts at Charlotte? Ciel? Pretty sure Ciel had perfect IVs, not sure about Charlotte.