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Now on to your question. I believe either way should work if you train your Pokemon well and give them good moves. What I usually do in those cases is train them all. It does take a bit longer, but it has the advantage of letting you choose the best combination of Pokemon for each gym, and then if you want to face the Elite 4 or battle against other players, you'll have tested all your options and know which ones should work best. You can even have different teams for single and double battles, which movesets that are most effective in either setting.