Shauntal

Type: Ghost
Preferred Pokemon: Liepard, Bisharp
Prize: $6000

Cofagrigus @ Level 48 Ability: Mummy - Shadow Ball - Will-o-Wisp - Grass Knot - Psychic

Jellicent @ Level 48 Ability: Cursed Body - Shadow Ball - Energy Ball - Surf - Brine

Golurk @ Level 48 Ability: Iron Fist - Brick Break - Earthquake - Shadow Punch - Curse

Chandelure @ Level 50 Ability: Flame Body - Fire Blast - Psychic - Shadow Ball - Payback

Shauntal may look hard on paper, but here team is really easy to defeat with a really good Dark-type. Unfortunately, Liepard isn't what I would consider 'good' but it works will considering her Pokemon's lack of speed. Chandelure's speed is the highest of this quartet, and is also the one you should be the most worried about. Liepard will outspeed Chandelure, and hit hard with Night Slash. Night Slash will take out Golurk completely, as well as Cofagrigus (Low HP + Super Effective = BOOM). Jellicent may be a problem since it's bulky, but Bisharp can handle it with Night Slash. I don't recommend Bisharp only because of Chandelure and the trouble it will face with Marshal and Grimsley. (Then again, Liepard won't stand much of a chance either! )

Grimsley

Type: Dark
Preferred Pokemon: Galvantula, Sawk/Throh
Prize: $6000

Scrafty @ Level 48 - Sand-Attack - Crunch - Brick Break - Poison Jab

Krookodile @ Level 48 - Earthquake - Crunch - Dragon Claw - Foul Play

Liepard (F) @ Level 48 - Night Slash - Attract - Fake Out - Aerial Ace

Bisharp @ Level 50 - X-Scissor - Night Slash - Metal Claw - Aerial Ace

So Grimsley is a weird guy. xD His team is pretty easy to defeat with Galvantula. Spam Bug Buzz on all of his Pokemon, considering you outrun them all. You might want to go with Thunder for Bisharp, just to do a bit more damage (and Bug Buzz no longer is super-effective on Bisharp). Scrafty's bulky, but won't last long with STAB'd Bug Buzz.

Marshal

Type: Fighting
Preferred Pokemon: Jellicent, Musharna
Prize: $6000

Throh @ Level 48 - Stone Edge - Storm Throw - Bulldoze - Payback

Sawk @ Level 48 - Stone Edge - Retaliate - Karate Chop - Grass Knot

Conkeldurr @ Level 48 - Stone Edge - Hammer Arm - Grass Knot - Retaliate

Mienshao @ Level 50 - U-Turn - Jump Kick - Rock Slide - Retaliate

So Marshall just may be the hardest on in the entire group. His Pokemon are ready for nearly all types, and struggle only with the likes of Jellicent and Musharna. Jellicent is a great candidate because of bulk and its ability to spread burns with Will-O-Wisp. Burns reduce Attack, which makes defeating Marshall's Attack-centric team so much easier. If you're looking for a more exciting approach, use Scald and burn his team with boiling water. :p And fear not for Jellicent. Conkeldurr and Sawk's Grass Knot will do minimal damage due to their horrible Special Attack.

Musharna also works because of it's Psychic-typing. The only real problem is Mienshao, since it knows U-Turn, but Musharna is bulky enough to absorb the blow and strike again with a STAB'd Psychic.

Caitlin

Type: Psychic
Preferred Pokemon: Galvantula, Bisharp
Prize: $6000

Reuniclus @ Level 48 - Energy Ball - Psychic - Focus Blast - Thunder

Musharna @ Level 48 - Reflect - Thunder Wave - Shadow Ball - Psychic

Sigilyph @ Level 48 - Shadow Ball - Ice Beam - Air Slash - Psychic

Gothitelle @ Level 50 - Thunderbolt - Psychic - Calm Mind - Shadow Ball

So Caitlin from Platinum's Battle Frontier has returned with a vengeance! She's pretty easy to beat too. Most of her Pokemon are fairly slow. Galvantula will be able to spam Bug Buzz while they quiver in fear. For Sigliyph, hit hard with Thunder. Bisharp also works well with Night Slash, but watch out for Reuniclus's Focus Blast.

 

Striaton City Gym

Gym Leader: Cilan, Cress and Chili
Type: Multi
Prize: Trio Badge, TM83 Work Up

Lillipup @ Level 12 - Bite - Work Up - Odor Sleuth - Helping Hand

Pansear/Pansage/Panpour @ Level 14 - Incinerate / Vine Whip / Water Gum - Fury Swipes - Work Up - Lick

These guys lead a weird gym. Depending on your starter, they change. If you pick Snivy, you'll face Chili, who has Pansear. If you picked Tepig, you'll have to battle Cress, who has Panpour. Finally, if you picked Oshawott, you'll face Cilan and his Pansage. Each trainer also has a Lillipup.

The best strategy to defeat Cilan, Cress and Chili is utilizing the gift Pokemon given to you in the Dreamyard. There's a young woman there willing to give you a Pokemon, depending on your starter. But instead of giving you the type-advantageous Pokemon, she'll give you the type-disadvantageous one. So if you picked Tepig, she'll give you a Pansage, which you can use against Cress's Panpour.

Basically, use that Pokemon to take them out.

Nacrene City Gym

Leader: Lenora
Type: Normal
Prize: Basic Badge, TM67 Retaliate

Herdier @ Level 18 - Bite - Take Down - Retaliate - Leer

Watchog @ Level 20 - Hypnosis - Leer - Retaliate - Crunch

A good Fighting-type Pokemon will come in handy here. I'd stay away from using Munna because of Watchog's Crunch. Due to poor type coverage, you can probably take them out with your starter. If that's not what you're looking for, try training a Timburr. They can be found near Pinwheel Forest.

Castellia City Gym

Leader: Burgh
Type: Bug
Prizes: Beetle Badge, TM76 Struggle Bug

Whirlipede @ Level 21 - Struggle Bug - Poison Tail - Screech - Pursuit

Dwebble @ Level 21 - Struggle Bug - Smack Down - Faint Attack - Sand-attack

Leavanny @ Level 23 - Razor Leaf - Struggle Bug - String Shot - Protect

All you need here is a really good Flying-type Pokemon. Tranquill and Swoobat will do the trick well, but watch out for Dwebble's Smack Down (or Faint Attack for Swoobat users.) Fire-types work well too, especially Special Fire. Pansage users will have fun in this gym.

Nimbasa City Gym

Leader: Elesa
Type: Electric
Prizes: Bolt Badge, TM72 Volt Switch

x2 Emolga @ Level 25 - Pursuit - Volt Switch - Aerial Ace - Quick Attack

Zebstrika @ Level 27 - Flame Charge - Volt Switch - Quick Attack - Spark

A Ground-type is really useful in this gym, especially with a Rock-type move for her Emolgas. Excadrill is a perfect candidate and can Slash Emolga away and focus on Zebstrika. Watch out for his Flame Charge!

Driftveil City Gym

Leader: Clay
Type: Ground
Prizes: Quake Badge, TM78 Bulldoze

Krokorok @ Level 29 - Crunch - Bulldoze - Swagger - Torment

Palpitoad @ Level 29 - Bulldoze - Muddy Water - Aqua Ring - Bubblebeam

Excadrill @ 31 - Slash - Rock Slide - Bulldoze - Hone Claws

A Grass-type Pokemon is preferable and a Water-type will help you take out Clay's team. The Grass-type is primarily for Palpitoad, but will suffice for Krokorok and dent Excadrill. Water will be more effective on Excadrill but won't hurt Palpitoad much.

Mistralton City Gym

Leader: Skyla
Type: Flying
Prizes: Jet Badge, TM62 Acrobatics

Swoobat @ Level 33 - Heart Stamp - Amnesia - Acrobatics - Assurance

Unfezant @ Level 33 - Quick Attack - Air Slash - Leer - Razor Wind

Swanna @ Level 35 - Bubblebeam - Aerial Ace - Air Slash - Aqua Ring

The best choice for Skyla is an Electric-type. With Chargestone Cave nearby, the game begs for you to use a strong Electric-type like Galvantula or Eelectrik. Rock also works, but Swanna may prove difficult.

Icirrus City Gym

Leader: Brycen
Type: Ice
Prizes: Icicle Badge, TM79 Frost Breath

Vannilish @ Level 37 - Mirror Shot - Frost Breath - Acid Armor - Astonish

Cryogonal @ Level 37 - Rapid Spin - Reflect - Frost Breath - Aurora Beam

Beartic @ Level 39 - Slash - Brine - Swagger - Icicle Crash

A Fighting-type is your best bet for Brycen. With Beartic's Brine, a slower Fire-type would have some trouble, as well as Rock-types. Rock-types may not even get pass Vanillish's Mirror Shot. Fighting-types have free roam over Brycen's team.

Opelucid City Gym

Leaders: Drayden (Black) / Iris (White)
Type: Dragon
Prizes: Legend Badge, TM82 Dragon Tail

Both teams are similar. However, Iris's Pokemon are all Females and Drayden's are all males. Iris's Druddigon also has Sheer Force instead of Rough Skin.

Fraxure @ Level 41 - Dragon Dance - Dragon Rage - Dragon Tail - Assurance

Druddigon @ Level 41 - Chip Away - Revenge - Dragon Tail - Night Slash

Haxorus @ Level 43 - Dragon Dance - Slash - Dragon Tail - Assurance

Ice-types are key! Beartic's Icicle Crash should suffice, but Vanilluxe's great defence will also help too. It's all up to you.... Or you could use your own Haxorus or Druddigon on them.