Trust me, do the friend safari thing at least for one Gyarados, you can have more than one if you want. I have a set for you, I call it the Elite Breaker:
Gyarados @ Mystic Water
Ability: Moxie
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
-Waterfall
-Custom Move [Grass]
-Dragon Dance
-Taunt
Alright, this set is made only to destroy the Elite 4. This set can make you beat both Kyla and Eduard without using any other Pokémon. Let’s talk about how it goes:
Kyla
Lead with Gyarados and use Taunt to avoid Smeargle to use any of its 4 status move. Then, use Dragon Dance once or twice. Having 81 base speed and having the maximum speed possible, Gyarados is 1 base speed baster than Kyla’s Blaziken and allow to outspeed it even with a turn of Speed Boost if you do a Dragon Dance. I highly recommend you to have 31 Speed IVs to make sure it works at +1. Custom Move [Grass] will allow you to hit Mega Slowbro for supper effective damage. Moxie is important here because Smeargle having an attack drop is useless and Moxie allow Gyarados to get enough attack boost to OHKO Mega Slowbro and Sylveon.
Eduard
Turn one, use Dragon Dance while Hiddpodown use Stealth Rock. To prevent Whirlwind, use Taunt turn two. Turn Three, use Dragon Dance as Eduard switch for Rotom-W. As you are at +2 Attack and Speed, you can outspeed even Choice Scarf variant and OHKO even defensive variant with Custom Move [Grass]. The rest of the team is OHKO by Waterfall, including Mega Cacturne, thanks to Moxie. With +2 Speed, you can outspeed any Pokémon except one, Excadrill. However, if the Excadrill is not Jolly, Timid, Naive or Hasty (and Exadrill is probably Adamant), you will outspeed it.
And that’s it! 2 E4 member who cannot fight back against that Gyarados. As I said, it is just a suggestion.