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To battle the Pokemon Heart Gold and Soul Silver Kanto gym leaders, you'll need to first defeat all the gym leaders in Johto and conquer the Elite Four. Once you've done that, visit Professor Elm and he'll give you the SS Ticket. Use this ticket to catch a boat from Olivine City in Johto to Vermilion City in Kanto. Note that you can battle the Kanto gym leaders in any order you choose, except for Blue, who must be battled last. Also, all four gym leaders on the second page can only be battled after you restore power to Kanto.
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Vermilion City Gym Leader - Lt. Surge
Lt. Surge uses Pokemon of the electric and steel variety. Use a combination of fire, rock, and ground type Pokemon to beat Lt. Surge. The Vermilion City gym has a bit of a puzzle to work out before you can challenge Lt. Surge. Inspect all the trash bins until you find one with a button to press, and then inspect an adjacent bin to find the second button. Pressing both buttons will shut off the force field and let you battle Lt. Surge. He uses a level 51 Raichu, two level 47 Electrode, a level 47 Magneton, and a level 53 Electabuzz. Once defeated he'll hand over the Thunder Badge.
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Saffron City Gym Leader - Sabrina
Saffron City's Sabrina utilizes a relatively small team of psychic type Pokemon. They can be bested with dark, ghost, or bug type Pokemon. To reach Sabrina you'll need to use the warp tiles scattered throughout her gym until you reach her. Her team consists of a level 53 Espeon, level 53 Mr. Mime, and level 55 Alakazam. Beat her to win the Marsh Badge.
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Celadon City Gym Leader - Erika
Erika, leader of Celadon City's gym, uses Pokemon of the grass type. Reaching this Pokemon gym leader is pretty straightforward. Proceed through the short hedge maze, defeating Erika's underlings along the way, and then you can take her on to earn the Rainbow Badge. She has a level 51 Jumpfluff, level 52 Tangela, level 56 Victreebel, and level 56 Bellossom. While the levels are high, you shouldn't have to worry as long as you have strong fire, ice, or flying type Pokemon.
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Fuschia City Gym Leader - Janine
Fuschia City marks the first major change in the Pokemon Heart Gold and Soul Silver Kanto gym leaders. While Pokemon Red and Blue featured Koga as the gym leader, you'll now have to fight his daughter Janine. The gym consists of invisible walls and a whole team of Janine lookalikes, so it may seem confusing. It's actually pretty straight forward, however, since you have to battle through all of the lookalikes before you get to the real Janine anyway. She uses Pokemon of the poison type, which are weak to psychic attacks. Her team includes a level 47 Crobat, two level 47 Ariados, a level 44 Weezing, and a level 50 Venemoth. In addition to psychic attacks, the Crobat will fall to electric attacks and the three bug Pokemon will succumb to fire. The Soul Badge is your reward.
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Cerulean City Gym Leader - Misty
Now you've reached the point where you have to complete a Kanto sidequest unique to Pokemon Heart Gold, Soul Silver, and the original Gold, Silver, and Crystal. Go to the Power Plant to the southeast of Cerulean City and you'll find a crucial part of the machinery has been stolen. Go back to Cerulean City and enter the gym to find the Team Rocket grunt responsible. He'll run away to the Nugget Bridge at the north of town. Follow him there, and after a quick battle he'll reveal he's hidden the machine part. Backtrack to the gym and find the piece hidden amongst the life rings, and then go back to the Power Plant to restore power to Kanto.
Head back to Cerulean City one last time. Misty still isn't in the gym, but you find her with her boyfriend at the end of Route 25 across the Nugget Bridge. She'll go back to the gym once you find her, so head back to the Cerulean City Gym to beat her and claim the Cascade Badge. She uses a water type team including a level 49 Golduck, level 49 Quagsire, level 52 Lapras, and level 54 Starmie. You can defeat all these Pokemon with grass or electric attacks, except for Quagsire, who is immune to electricity.
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Pewter City Gym Leader - Brock
Now that you've restore power to Kanto, head to Lavendar Town and enter the radio tower. The owner will be so impressed with your work that he'll hand over a special chip for your PokeGear radio. It lets you tune into Kanto radio stations. Find the Poke Flute station and use it to awaken the Snorlax blocking the path to Diglett's Cave outside Vermilion City. With access to the cave restored, you can pass through to reach the final three gym leaders.
You'll find rock and ground type user Brock in Pewter City gym. Just head north from where you came out of Diglett's cave. Use water or grass attacks to take down his level 51 Graveler, level 51 Rhyhorn, and level 54 Onix. Stick to grass moves for finishing off his level 53 Omastar and level 52 Kabutops, and you'll earn the Boulder Badge.
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Seafoam Islands Gym Leader - Blaine
Here's another unique change for the Pokemon Heart Gold and Soul Silver Kanto gym leaders. In the years since Pokemon Red and Blue, a volcano has destroyed the settlement on Cinnabar Island. When you arrive on the ruined island, you'll see a lone figure standing by some rocks. Talk to him and he'll reveal himself as Blue (Gary Oak from the anime), the final gym leader. He'll tell you to find him in the Viridian City gym once you've collected all the other badges, and then he'll fly off.
To find Cinnabar Island's former gym leader, Blaine, you'll need to surf to the east to the Seafoam Islands. Blaine and his minions use fire type Pokemon, so bring strong water type Pokemon to defeat his level 54 Magmar, level 54 Magcargo, and level 59 Rapidash. As a reward you'll get the Volcano Badge.
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Viridian City Gym Leader - Blue
Blue, the annoying rival character from Pokemon Red and Blue, has apparently gone on to great things. You can find him in the Viridian City Gym, where you'll have to battle a few underlings and navigate a maze of zip-tiles to reach his podium. Gary uses an electric Pokemon team. Use a fire, psychic, or ice type Pokemon to defeat his level 55 Exeggutor. Take down his level 56 Machamp with a flying or psychic type Pokemon. Douse his level 58 Arcanine with a water type Pokemon. Defeat his level 58 Rhydon with a grass or water type Pokemon. Finish his level 52 Gyrados with electric, rock, dragon, or ice Pokemon, and complete the job by frying his level 60 Pidgeot with electricity. Your reward is the Earth Badge.
All that stands between you and being the Pokemon champion is to rechallenge the Elite Four and defeat the ultimate Pokemon trainer, Red, at Mt. Silver. You can also find new legendary Pokemon, such as Zapdos and Moltres, now that you've collected all 16 badges.