I would say that fire is the most effective. Jynx is a Ice and Psychic type. Fire, Fighting, Rock, Steel, Bug, Ghost, and Dark will Be the most effective.
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Before the 8th gym, I trained my Pokemon in the Ice Path and on route 45 before beating the leader. It helps to catch a swinub or jynx in the ice path because dragon Pokemon are weak to ice. Claire's Kingdra is really tough. because of its water-dragon typing, it has lost its weakness to ice. However, I took it out with strong electric typed attacks from my ampharos.
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trade it
Go to Cerulean City, and go to the house next to the Pokemon Center. Talk to the old man and he will trade his Jynx for your Poliwirl. *Your Jynx will be the same level as your Poliwirl.
The following Pokemon in Pokemon yellow cannot be caught, Weedle, Kakuna, Beedrill, Ekans, Arbok, Meowth, Persain, Koffing, Weezing, Jynx, Electabuzz, and Magmar. You will need to trade from red or blue and to get the jynx's from red and blue you'll need to trade a polywhirl for a jynx in cerulean city.