Pokemon FireRed & LeafGreen: Gym Leaders & How to Beat Them
Brock of Pewter City
Type: Rock
Prizes: $1400, TM39 Rock Tomb, Availability to Flash
Suggested Pokemon: Mankey
 Geodude @ None Level 12 Ability: Rock Head - Tackle - Defense Curl |  Onix @ None Level 14 Ability: Rock Head - Tackle - Bind - Rock Tomb |
Brock, being the first gym leader, is also the easiest. His team can be taken down by a Water-type or a Grass-type. Thing is... there aren't any of these available before him. Mankey, on the other hand, can put a hurting on Brock's team, despite Onix's excellent Defense. Your starter will also help, especially Squirtle and Bulbasaur, but Charmander gets access to Metal Claw at level 13, so there's no worries there.
Misty of Cerulean City
Type: Water
Prizes: $2100, TM03 Water Pulse, Availability to Cut
Suggested Pokemon: Paras, Pikachu
 Staryu @ None Level 18 Ability: Natural Cure - Tackle - Recover - Harden - Water Pulse |  Staryu @ None Level 21 Ability: Natural Cure - Rapid Spin - Swift - Recover - Water Pulse |
Paras is a great candidate for Misty, as well as Pikachu. With availability to Bug AND Grass type moves, it's the perfect counter for Starmie. Stun Spore or Poisonpowder it, then spam Leech Life or switch into Pikachu to zap it.
Lt. Surge of Vermillion City
Type: Electric
Prizes: $2400, TM34 Shock Wave, Availability to Fly
Suggested Pokemon: Geodude
 Voltorb @ None Level 21 Ability: Soundproof - Shock Wave - Tackle - Sonicboom - Screech |  Pikachu @ None Level 18 Ability: Static - Quick Attack - Double Team - Thunder Wave - Shock Wave |  Raichu @ None Level 24 Ability: Static - Quick Attack - Double Team - Thunder Wave - Shock Wave |
Geodude shouldn't have much of an issue here, considering none of their attacks can really damage it much. It learns Magnitude at level 16, and that's all you're going to need to defeat Lt. Surge.
Erika of Celadon City
Type: Grass
Prizes: $2900, TM19 Giga Drain, Availability to Strength
Suggested Pokemon: Fearow, Pidgeotto, Vulpix/Growlithe
 Victreebel @ None Level 29 Ability: Chlorophyll - Giga Drain - Stun Spore - Acid - Poisonpowder |  Tangela @ None Level 24 Ability: Chlorophyll - Poisonpowder - Constrict - Ingrain - Giga Drain |  Vileplume @ None Level 29 Ability: Chlorophyll - Sleep Powder - Acid - Stun Spore - Giga Drain |
Erika's team isn't very defensive (besides Tangela) and can be taken down by any strong Flying-type, like Pidgeot and Fearow. Fly will take down just about every one of Erika's team. Vulpix and Growlithe will also deal with Erika well.
Koga of Fuschia City
Type: Poison
Prizes: $4300, TM06 Toxic, Availability to Surf
Suggested: Hypno, Kadabra
 Koffing @ None Level 37 Ability: Levitate - Sludge - Smokescreen - Toxic - Selfdestruct | Koffing @ None Level 37 Ability: Levitate - Sludge - Smokescreen - Toxic - Selfdestruct |  Muk @ None Level 38 - Minimize - Acid Armor - Toxic - Sludge |  Weezing @ None Level 43 - Tackle - Sludge - Toxic - Smokescreen |
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Koga's team is unbalanced, but his Pokemon are sturdy to non-supereffective attacks. Bring along a strong Psychic-type to take him down. Hypno works by hypnotizing them and using Dream Eater or can just attack head on with Psychic. Alakazam is even better, but Hypno is sturdier and easier to obtain.
Sabrina of Saffron City
Type: Psychic
Prizes: $4300, TM04 Calm Mind, Availability to Rock Smash
Suggested Pokemon: Haunter/Gengar
 Kadabra @ None Level 38 - Psybeam - Reflect - Future Sight - Calm Mind |  Mr. Mime @ None Level 38 - Psybeam - Barrier - Baton Pass - Calm Mind |  Venomoth @ None Level 38 - Psybeam - Gust - Supersonic - Leech Life |  Kadabra @ None Level 38 - Psychic - Recover - Future Sight - Calm Mind |
Sabrina's pretty challenging, considering Psychic is a great offensive type. Ghost is your only hope of beating her, so use a Haunter or Gengar to smack her team around. Venomoth will need a Fire-type or Rock-type, but besides that, you shouldn't have much of a problem.
Blaine of Cinnabar Island
Type: Fire
Prizes: $4700, TM50 Overheat, Availability to Waterfall
Suggested Pokemon: Golduck, Gyarados
 Growlithe @ None Level 42 Ability: Intimidate - Bite - Take Down - Roar - Fire Blast |  Ponyta @ None Level 40 Ability: Flash Fire - Stomb - Bounce - Fire Spin - Fire Blast |  Rapidash @ None Level 42 Ability: Flash Fire - Stomb - Bounce - Fire Spin - Fire Blast |  Arcanine @ None Level 47 Ability: Intimidate - Bite - Take Down - Roar - Fire Blast |
Blaine's team is basically the same two Pokemon just stronger. A Gyarados will take care of him completely, as well as a Golduck. Rain Dance sets are great as well, and dampen his Pokemon's ability.
Giovanni of Viridian City
Type: Ground
Prizes: $5000, TM26 Earthquake
Suggested Pokemon: Gyarados, Starmie
 Rhyhorn @ None Level 45 Ability: Lightningrod - Take Down - Scary Face - Rock Blast - Earthquake |  Dugtrio @ None Level 42 Ability: Sand Veil - Slash - Sand Tomb - Mud Slap - Earthquake |  Nidoqueen @ None Level 44 Ability: Poison Point - Poison Sting - Body Slam - Double Kick - Earthquake |  Nidoking @ None Level 45 Ability: Poison Point - Poison Sting - Thrash - Double Kick - Earthquake | Rhyhorn @ None Level 50 Ability: Lightningrod - Take Down - Scary Face - Rock Blast - Earthquake |
Gyarados will be able to sufficiently take down Giovanni, though a special Water-type like Starmie will also work great, especially for Nidoking and Nidoqueen.
