I haven't played HG in a long while, and the battle had to be at least two years ago, but I still remember the horrors of facing her Miltank. I spent a good half hour grinding its health down; in the end, it wiped out my entire team except for my Flaaffy. I finally defeated it (after using up almost all of my potions) by paralyzing it and managing to KO it with some sort of electrical attack.
Most recently, in SS, I had no problems with Whitney :3 I got lucky with a critical hit using my Pidgeotto...? Dunno why it worked.
Chuck was horrible, though... T_T I couldn't beat him for a while, since most of my Pokemon were 6-7 levels beneath his at the time... I had two flying-types and a grass-type, and I still had problems, even when they leveled up to the high 20's.
At most, it got me 2 tries to beat Whitney. The first time I was careless, I though my ghastly could take care of her milktank until I learned it had scrappy. I won the rest on my first try, easily or with an improbable plan. For example, for Clair, when she's down to her kingdra, I used my entei. It used protect and occa berries to survive all its hydro pumps, while wasting its pp with its pressure. Then it used Strength multiple times to finish it off.
Whitney was tough, but wasn't THAT tough for me to handle (I got a bit of leveling up done before-hand). In all honesty, I had some difficulty with Jasmine. Even with my Typhlosion out, who was much higer-leveled than anyone else on my team, her Steelix gave me a lot of trouble, just because it has such a high defense (and possibly HP?). Usually she doesn't give me much trouble, but in this file she did ._.
Clair was horrible. I had a Dragonair on my team at this point, and it still took me over a week to beat her...
Strangely though, my cousin beat her on her first try, probably something to do with me leveling up her Pokemon for her