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Gyarados is far better than I thought
My Gyarados performs especially good against Vaporeons, with DT/Outrage, an Outrage can take a great amount of Hit Points from a 2xxx CP Vaporeon. Seriously, Gyarados isn't that useless.
My Gyarados is level 20.
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What I do to save potions and time is only choosing the first (maybe the second one too) pokemon for the figth based on optimal counters, then only figth the first (again, maybe the second one too) pokemon, then run away, rinse and repeat until those are out of the gym, then do the same for the rest until you cleared the gym, that way you take less damage, from each figth and take less time to defeat (and kick out) each pokemon and clear out the gym.