I thought everyone said she was supposed to be hard? Why couldn't she beat a gimmick Shuckle?
None of her Pokemon knew taunt or any phazing move that could cancel out the Shell Smash Contrary defense boosts. Anyone could set up on her and she'd be none the wiser.
To be fair, I did lose once with Shuckle, but it was to Lucian and not Cynthia. Lucian's Alakazam had Magic Guard, so I had no option but to stall until Alakazam ran out of PP and used Struggle. Unfortunately, Alakazam was able to set up three Nasty Plots and got a critical hit on Dazzling Gleam. So I had to temporarily switch Rest with Flash for Lucian's battle.
Every other Pokemon was easy to defeat. Bertha's Hippowdon and Cynthia's Milotic took a while to whither down, but they weren't necessarily difficult.