Brock

explain

Easy to complete league with Charmander, Bulbasaur is always harder,

easier to complete the first 4 gyms were easy for bulbasaur (actually the fifth one was easy with an evolved form of him) since rock/water is weak to grass, electricity does weaker dmg to grass, and by the fourth gym (grass) he was most likely ivysaur, so poison attacks gave him type advantage, and nulled the supereffective moves from the fifth gym leader, poison type, honestly, you would only had trouble on sabrina and blaine, for obvious reasons. and the first two members of the elite four had water or a rock type that did 2x dmg to charizard, (though the ice types of the first member would give venusaur trouble as well) and lance had an aerodactyl which is rock flying, giving troubles to the both of them. bulbasaur is actually the easiest to complete the game with if you look at it closely

That's why you use fucking Charizard