Ok, I've only played them all at E14, so I don't know about pre-field difficulty.
Aya is quite easy. Underleveled, double battle and starts with a total non-threat in Nidoqueen. You can just double-attack the other ones. And once Gengar is gone, she's done.
And last time I respected Charlotte too much, I grinded with Aya to have a team at 63-65 and ended up crushing her. I used one hyper potion and lost 3 pokes, one Flygon that took a critical Eruption AND Darumaka's first attack, one Arcanine without Flash Fire (had forgotten to use the capsule) that I killed with an EQ, and another Flygon (I have 2) that went down when she was down to one 'mon.
Charlotte is just a spook story. EQ and Rock Slide spam rek her.
I'd say the toughest is Noel, who was harder on my second playthrough than the first. Also props to Kiki (hits effin hard, you have to KO her before she hits, that Medicham is the best momentum-changer ever), Serra (kinda odd team, but not bad at all) and Shade (the toughest, well, most damaging, one in my sub-480 run, where I've fought everyone before Bennett and Serra).
Samson also hits like a maniac, but he can lose to very underleveled teams if played correctly.