I don't remember exactly how I managed to beat her the first time, but it was with a Croconaw w/out Ice Punch. Nowadays, I use Heracross, around 18-25. I may include an Oddish and a Totodile/Croconaw w/ Ice Punch.
Looking back, w/ Stomp and the perpetual strength of Rollout, it would be difficult to defend. W/ Milk Drink and Attract, it would be difficult to attack. Even Clefairy can give you a run for your money if unprepared. But because of Whitney, I was able to rever it and use it against Lance's Dragonite. Every time I face her, I don't hold back. Thanks for giving her a thread.