In RBY, I must admit that the Gym Leader I had the most trouble with was ironically, Misty. Her Starmie sure packed a punch, and Pidgeotto wasn't fast enough to kill it, the wimpy Pikachu I caught in Viridian inflicted little damage, and alas, my poor Charmander was out in one hit! Even playing the Games with Bulbasaur as a starter was difficult. Vine Whip didn't hit hard enough, and I didn't always get Razor Leaf in time (leveling up takes time, and I'm impatient...). I've heard however, that using Wartortle is very useful.