Pokemon HeartGold & SoulSilver :: Gym Leader Guide
While walking through your first cave, try to grab a Geodude, especially if you choose Chikorita as your starting pokemon. As long as your Geodude is around level 13 or 14 and knows Rock Throw, you will have a pretty easy time with this bird freak.
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If you took Cyndaquil as your starter, you will have an easy time at this Bug gym. Again, if you started with Chikorita, you should be leveling up that Geodude. At around level 19, Geodude should be able to withstand most of Scyther's hits and hit back enough to make it an easy fight.
Here's hoping that your starter pokemon is female. Whitney likes to charm and attract your pokemon, which is very annoying. Keep leveling up your Geodude, since you will be using him again to soak up most of the hits, especially from Miltank.
If you spent the time and pokeballs to capture an Abra and level it up, you will be rewarded here in an otherwise difficult gym. Luckily, Morty doesn't use true ghost types and all of his pokemons have a poison type that is weak against psychic. Just be sure that you level up enough so that you can hit first! Gengar's Shadow Ball will ruin your day. Totodile trainers can rely on Bite as well.
Keep leveling that Kadabra, because you will be using him again here. As before, speed is the difference maker as Kadabra will most likely not survive more than two hits of anything. Take advantage of the long surf to the city to focus on Kadabra.
If you are a Cyndaquil trainer, you should have no problem here! Your Geodude/Graveller should be able to take out the two Magnemites without any hassle. For Steelix, use special attacks: water, fire, grass, almost anything will put a good dent in its health.
Hopefully you have been training up a Mareep this whole time (you should, they are good). Any electric attacks will take care of Seel and Dewgong. Chikorita trainers could also try using their grass pokemon but Ice Shard makes it a risky bet. Totodile and Cyndaquil trainers will be able to handle Piloswine with relative ease. Use Kadabra to finish off anyone if needed.
Here's a real hard fight. If you started with Totodile, you should be sure to give it an ice attack. You should have a water pokemon trained up regardless for all your surfing needs. Make sure that pokemon has an ice attack. The electric pokemon you used for the last gym will be useful here for Gyarados and Kingdra, but will be completely useless against Dragonair. Kadabra's Psychic will be able to knock off a good chunk of health before going down. All of Clair's pokemon hit hard, so don't expect anyone to stand for too long and bring lots of revives to the fight.
This psychic trainer does not use many pure psychics. You can hit Xatu and Slowbro with electricity and beat Jynx and Exeggutor with fire. Totodile trainers will have access to a dark attack and if you have been training up Mareep, you should have the bug-move Beam Signal at your disposal as well.
If your Kadabra is at a good level, you should be hitting first and taking down most of Koga's pokemons in one hit. Only Muk and Forettress will give you trouble. Muk needs two psychic blasts to take down and hits hard so that your Kadabra might not be able to stay in battle. Forettress can be taken down by a good fire attack. If you don't have one handy, use special attacks. Since Koga is a poison user, it makes sense to carry lots of Full Heals and Full Restores with you.
Bruno is still a fighter and still weak to Psychic. If Kadabra gets knocked out, you can also use Flying type attacks in a pinch. Only Onix is different, but you can use almost anything else to take down that rock pokemon.
Fighting pokemon are not necessary here against this Dark lover, but it will help. Especially against Umbreon, which will be able to take a good solid hit of almost anything. If you have Mareep and its Beam Signal, you can use it against Umbreon. Vileplume will fall to fire, psychic and flying attack and Gengar is also weak to psychic and, luckily, does not have Shadow Ball. Murkrow and Houndoom will easily go down to electric and water attacks respectively.
Dragons are really tough and they make Lance a great champion. Even compared to Clair, this will be your hardest fight yet! Again, have a good electric type for Gyarados. It will also be handy in finishing off Aerodactyl, Charizard and Dragonite since they also have a flying type. Aerodactyl and Charizard also falls to a good surf from a water pokemon or a rock slide from your Graveller. You should be using Graveller when you can and saving your water pokemon for its ice attacks and use it against Lance's two Dragonairs. Expect to use your max revives here. Just like Clair, Lance's dragons hit hard no matter what you throw out.