I am currently in Mistralton city and I have challenged, battled, and have been defeated by Skyla multiple times.. I have had no problem with defeating Swoobat and Skarmory, but once it came to Swanna, I never had the chance to battle much as she would take each of my Pokemon out in one move. Any fellow trainers who could help???
Electric type moves do are super super effective against swanna, so I recommend using a Jolteon, since it can out-speed Swanna and take her out with one hit. I did this myself when playing black 2.
Name: Magnemite Availability: Early Game, Virbank Complex, 25% (Inside) Stats: Has a great Special Attack stat, with decent overall bulk to back it up. Typing: Steel/Electric is arguably the holy grail of defensive typings, with resistances to everything bar four types. Ground and Fighting types are rather common though. Offensively, this typing is mediocre, but is saved by Magnemite's stats and movepool. Movepool: Magnemite comes with Sonicboom, a godsend earlygame that can 2HKO everything. Aside from that, it has Thundershock, and later Mirror Shot. Magnemite doesn't get any good STAB moves until Flash Cannon, but it can hold on with STAB Volt Switch, Thunder, and a tutored Signal Beam. Major Battles: Magnemite steamrolls over Roxie, Burgh, Skyla, and Marlon by virtue of typing alone. It helps a lot against Drayden, but needs to watch out for coverage moves such as Earth Power.
source tldr Magnezone is the best electric Pokemon in the game because it's also steel type and has superior special attack.
Your best option is to catch a Zebstrika which is available in the adjacent Route 7 and train it a bit using Audino in the shaking spots. Zebstrika's natural high speed and typing make it super easy to defeat Skyla.
It's available at levels 34-36, and it learns the move Discharge at 36, making it super strong against Skyla. Levelling it up a little bit can ensure OHKOing the troublesome Swanna.
The reason I suggested this over every other Pokemon is that it's catchable in game and literally on the route right best to Mistralton. And it's fast enough to get the job done. Good luck!