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Good chance that you do not have Electric type Pokemon yet, so you will have to go with something else. Onix can be obtained early by trading a Bellsprout and raising it until it learns Rock Throw, or do this with Geodudes obtained near New Bark.

Fire and Flying types will work here, and are the best types to use if the Pokemon is strong enough.

Miltank is the only problem because it knows the deadly Rollout move and uses it often. It will also use Milk Drink when it's nearly fainting. Keep attacking with your most powerful moves. Try using Sand-Attack if things don't work out for you.

Psychic type moves are good to use here, but Psychic Pokemon will die fast. Ground type Pokemon and moves are alternatives here.

Steel is immune to Poison but weak against Fire. Strong Fire moves and a speedy Pokemon are key to taking out Steelix.

Psychic and Flying type moves are key to this battle, given that your Pokemon can withstand the onslaught of punches from these two. Electric and Leaf is also an alternative to Poliwhirl.

Electric moves on Seel and Dewgong, since Water now nullifies Fire effectiveness on Ice. However, that's not the case for Poliswine, so attack away with Fire moves.

All her Dragonair are a pain to go through, but Dragon and Ice moves will wipe the floor with them. Kingdra is the worst of them all, since Dragon nulls the effectiveness of Electric moves. Ice and Electric moves are the best to use, even if they don't do much. An all out assault with strong moves will be the best bet.

Leaf and Water type moves will easily take out Brock's Pokemon. Solarbeam/Razor Leaf recommended.

Misty's Pokemon are all greatly weak to Electric type moves, with the exception of Quagsire. Leaf moves will be needed to take it down.

All his Pokemon are weak to Ground type moves and have low defense. Magneton should have Fire type moves used on it instead.

Powerful Fire type moves are the only thing needed here along with a couple of Full Heals.

Like her father, Koga, in R/B and in the Elite 4, her Poison Pokemon will go down with Psychic or powerful physical moves.

The new Dark type Pokemon are immune to Psychic attacks, and their Dark-type moves are greatly effective to Psychic Pokemon. Use that to your advantage.

Like in R/B, strong Ground and Water type moves will wipe out his party.

The last gym is a mix of sorts. Use Electric moves on Gyarados and Pidgeot. Use Water moves on Rhydon and Arcanine. Use Fire, Dark, or Ice moves on Exeggutor and Alakazam. It is a very difficult battle if you're not prepared.