Hmmm, I think grinding as you go is the answer, then.
Usually the first E4 member is 2-3 levels higher than the last gym leader, but aiming for the first E4 member is not a good thing because the Champion can have 10 levels over that.
All my questions are because, when I play, I'm usually just a tiny bit overleveled. I'm not a player who takes on challenges underleveled. But I don't understand how there are people who can easily level up. Maybe they just play more than me?
The only region I found it was easy to level up was, ironically, Unova, with the new exp system. Audinos make grinding much less of a pain. I wonder what XY will have in terms of grinding.