I dunno if I've just had absurd luck or what, but Cheren and Roxie were both cakewalks for me. Beat the entire first gym undefeated with Azurill, and the second identically with Magnemite.
In terms of Roxie, you used Magnemite so she was pretty much handicapped, she had no way to use her Poison moves.
For me, I've beaten up to Clay and it's been:
Cheren-One loss, but I battled him right after I defeated those other two. The next time I won I believe, but it was down to Tepig or Psyduck and whichever one had little health left, in the Red.
Roxie-Another close battle, Pignite and Psyduck used.
Burgh-I think it was close, used Pignite, Psyduck, and Sandile.
Elesa-I was able to win with two Pokemon only, Heracross's Attack is probably why, Pignite lost at some point, and I had Sandile ready to use.
Clay-Another close battle, Pignite, Krokorok, and Golduck used, the first two fainted at some point, Golduck defeated Excadrill with around half it's health still or something.
I probably would have lost a few times against most, but I always just restart the game whenever I lose or things aren't going good, I just don't want to bother with navigating through the gym again.