I am in my first run of the game, currently grinding for Terra.
In terms of characters so far, I don't like Terra, she is awfully random, but I would put Saphira and Luna a bit higher, I like them. I despise Blake so far, so I wouldn't put him top tier, maybe Zel a little higher.
In terms of gym battles, Charlotte was hard, but I would't say that hard, because I destroyed her field and was pretty ok after that.
I think Shelly and Aya gave me the hardest time, maybe because I wasn't used to the difficulty and strat needs for this game.