Given the lack of great defenders to form a solid gym (it seems that everything is winnable) and hoping that the legendaries are included soon in the gyms, I decided to base my strategy on looking for good defenders and harness them taking advantage of the climatic impulse in their movements.
When a Pokemon move is being influenced by its corresponding weather, it will deal approximately 20% more damage. When calculating move damage, Pokemon type has nothing to do with bonuses.
Pokemon with double weakness do not include them because it is very easy to counter them (Dragonite, Tiranitar, etc)
Having said all this, these are for me the best advocates driven by the weather:
Rainy: Vaporeon (Wg / AT), Lanturn (Wg / Tb), Muk (In / Bz) Swalot (In/IB)
Sunny: Steelix (DT/Ea)
Cloudy: Donphan (Ct/DG) Tentacruel (Ac/Bz)
Do you have any to add?