Get them all around level 69 (70 is dangerously high and they might stop obeying during the fight). Charlotte has a higher leveled ninetales at 74, so you need all the levels you can have.
If you play with potions and that (Some don´t, just sayin´^^), try to set up Dragon Dance right at the beginning three times (two times if it has moxie) and heal with that Fullheal or something each time you get confusion, because that´s what that Ninetales at the beginning likes to do ^-^
As soon as you fainted that first Ninetales (Solarbeam^^), throw in Golem and spam Earthquake while Gyarados takes out everything else. As soon as the other Ninetales appears, you´ll just make that stab Aqua Tail (Well, I dunno if it´ll be oneshot without moxie as ability ^^). Either way you´ll be able to deal quite some damage to her team before Gyarados faints. She also has that fire-type rotom, which should be disposed of quickly with another aqua tail, because that thing is her only real type-coverage against gyarados,
from what I´ve seen at least.
She also has a delphox and for that thing you might need your Scrafty, if your Gyarados faints somewhere in between.
If your Gardevoir has the right stats, you might as well try to tank a few special hits, heal up and set up Double Team and Calm Mind, if you can (Mine is EV-tained in sp.def, so I´m not 100% sure ^-^)
Maybe try Swaggering that first Ninetales so it can´t use Solarbeam? Because I remember its attacks being a bit painful, at least it was a little painful for my gyarados ^^ Either that or confuse both Pokemon with Crobat, if you can do it.