Personally, I like how hard Char was, even now that I have had to try about 13 times to beat her. Getting those predictions up and trying out your strategies, keeps the brain sharp.
Even tho I like to play with my favorites, like Karen has taught me, Charlotte might have been the first time I changed up the team. Galvantula and Whimsicott weren't going to help me much) and I tried some different tactics and other Pokemon from my PC in an attempt to defeat her. Also, I don't save in front of the leader, get beaten and soft reset, I black out and try again, because exp is exp. THEN I'll train them up some more and after I'm bored of grinding, I try again.
(What's the deal with people being so against fainting their Pokemon? It's not like they instantly start to hate you and turn against you)
I remember with Serra I led off with Emolga against her Cloyster, knowing I'd outspeed it for a Nuzzle, and hoping I'd get that Paralyze so it wouldn't wreck me with Icicle Spear/Rock Blast. It didn't work most of the time but when it did I was happy it worked and finished it off with Electro Ball.
Serra being not so hard but still challenging doesn't matter as much, since there's another Ice type specialist who has wrecked me the most out of any battle I had. Having Terra derping around after that and being able to beat her in 1 try balances it out again.
Keep it how it is, tho Terra might be slightly more of a challenge to me. Bertha from the Sinnoh Elite 4 was pretty much the same to me as Terra.