Koga was very easy for me, I had a Hypno with Psychic and took down his whole team with it. Brock, Misty, Erika, Lt. Surge, Blaine and the other gym leaders were easy. (FR/LG btw). In Blue, I had trouble with Brock because of the ground weakness(chose Bulbasaur). The other ones I used a Dugtrio to kill them all.