I never played Gen II, so my only experience with Claire and Whitney was in HeartGold, where my team was pretty nicely levelled, if not over-levelled. Neither trainer proved to be particularly difficult, though Clair is definitely the more difficult of the two. I remember Whitney was surprisingly tough for me to get through, but not to the point where I actually lost the match. She just KOd one of my Pokemon, which I didn't think she'd be able to do. It wasn't as though I was losing to her over and over. Kingdra wasn't too much of a challenge for me at either because my starter was Chikorita, and by that point he had evolved into a bulky Meganium that was able to take pretty much anything Kingdra threw at it and hit back with some decent Grass moves. Meganium was always surprising me with how much punishment it was able to take. But then again, maybe I was just over-leveled.