Recommended Team: Ground-types (Sandile, Sandshrew etc.), Magnemite or other Electric-types with strong Normal-type moves
Time to crack open a can of ass-whooping... The New Nimbasa City Gym is east of the Ferris Wheel. Heal and prepare your party, then head there. Enter and you'll be stopped by Clyde. He'll give you some tips and throw you a Fresh Water . To reach Elesa, this gym's leader, you must reach the end of the catwalk.
The catwalk acts as a bridge of sorts and has three Beauties placed along it that you must defeat before you can reach the final battle. This Gym has a focus on Electric-type Pokemon, so make sure you have Pokemon with types that trump Electric and don't have any (or many) that have a weakness to Electric.
When you're ready to, step onto the runway . When you get close enough to a Gym Member, the spotlight will shine on them. When this happens, they move over to you, with the spotlight following, and challenge you to a battle. The first battle's against a woman named Tesla.
Excuse the odd names that the trainers are called; they're tributes to three great scientists with two of them significantly contributing to fields relating to electricity, so it's only natural. Nikola Tesla was a major contributor to the development of the AC (Alternating Current) system. Yay for including science references in a game!
Gym Member Battle: Beauty
Once you've defeated Tesla, she'll spin around and walk off. If you want to heal your Pokemon now or at any time, feel free to do so. When you're ready to, continue to the next Beauty, Fremy. Fremy was the discoverer of Fremy's Salt, a useful chemical in some spectroscopies.
Gym Member Battle: Beauty
Fremy will spin and strut away. Continue to the last of the Gym Members, Ampere. Andre-Marie Ampere is one of the founders of the field Classical Electromagnetism and has an SI unit named after him, the Ampere, which is a measurement of current.
Once she's defeated, she'll stride away and leave you with only the Gym Leader, Elesa, to face. The camera will tilt so that she's in view. Three spotlights will focus on her and the screen behind her will show her face. When you're good to go, move up and talk to her. After conversing for a while, the battle will begin.
Gym Leader Battle - Elesa
This battle is rather annoying. Going in with Pokemon below an average of around Level 25 means that you'll have next to no chance unless you're using them very well and have some luck on your side. Above that and you should be able to make it through, if you play your cards right.
Getting good type match-ups in this battle can be difficult and combined with Volt Switch's nature, forms the reason why this battle is painful. Using Ground-type Pokemon is ideal, except against Emolga. Electric-type moves won't damage you, but when you're versing Emolga, Ground-type moves won't damage it.
Magnemite is surprisingly good against Elesa's Pokemon too as all moves, with the exception of Zebstrika's Flame Charge, are weak when used on Steel-types. Obviously, using Magnemite's Steel- or Electric-type moves isn't going to do a lot, so you need to be using its Normal-type moves instead.
Although Grass-types can seem appealing against Electric-type enemies, don't be fooled. Zebstrika's Flame Charge and Emolga's Aerial Ace mean that you shouldn't be using Grass-types against either of them. Only being able to use them against one of three Pokemon means that they aren't really that great.
Because you need specific Pokemon types for specific match-ups, Volt Switch can be a serious pain. It not only does a horribly amount of damage, but the Pokemon that uses automatically switches out with another Pokemon in Elesa's party without missing out on a turn.
This means that it's easy to be caught out and suddenly go from a strong type match-up to a really weak one. To counter this, you need to defeat the party as quickly as possible, taking out Zebstrika so that you can use Magnemite against the other two, or Emolga so that you can use Ground-types against the other two.
In the meantime, it's best to use Pokemon that are neutral to these Pokemon. This means good old Normal-types, or types with only slight disadvantages (Fighting etc.). Electric-types with strong Normal-type moves also work well in this role, especially against the powerful Volt Switch which doesn't threaten Electric-types.
Through all this, you should be watching out for Elesa using one Hyper Potion on one of her Pokemon when its HP is low. Zebstrika also has a Sitrus Berry that will restore his HP when it's low, so obviously watch out for that too.
Meanwhile, you may want to use some of your weaker Pokemon as fodder to allow you to use Revives and Super Potions on any of your strong Pokemon that succumb and faint. Make good use of moves that change the status of enemies, especially to sleep.
Continue using neutral types until either Zebstrika or Emolga are defeated, then switch in the Pokemon that's strong against the other two of Elesa's party and wreak havoc. From there, it's a battle of pure power. Plow through them until there's nothing left of Elesa's party.
Elesa will then go on to glamorously congratulate you on winning and present you with the Bolt Badge! She'll let you know that traded Pokemon up to Level 50 will obey you and will then give you that insanely annoying TM, TM72! Now you can teach Volt Switch to your Pokemon!
Continue conversing until she feels like shutting up, then leave the Gym. A lightshow will start and she'll come and escort you down the runway. Strut your stuff! Once you reach the end, feel free to spin around like the Beauties did. Soak in the glory! Now let's leave.