Scyl

Nah, no Hyper Voice move tutor in Sheridan, but Pixilate Swift/Quick Attack/Even Facade is still pretty good.

I don't think you get access to Calm Mind in the game, period. And even if you did, Sylveon also gets Calm Mind, so...

I'd say Sylveon is the better of the two, at least at this point in the game. It has greater bulk overall, since Clefable only has like an 8 point difference in Physical Defense; They have the same base HP and Sylveon has waaaaaaaaay higher base Special Defense. You'd also have to keep Clefabled as a Clefairy until it gets Moonblast... Which is at level 47 and the rest of the moves that make Clefable good aren't accessible until waaaaaaay later. Where as Sylveon can have Pixilate to give it much needed Stab.

Warm Welcomes and Savage Salutations. uwu.

I mean, if you want to have the easiest time, you can rock Torchic with Speed Boost. (But that's no fun, now is it.) Fennekin is pretty good, it gets a great learnset as a Delphox boasting great coverage by level up and Shadow Ball by heart scale. Mudkip is alright, it's solid enough. Bulbasaur is pretty bad due to not getting any good moves by level up, and you can easily get a Budew early game, which just outclasses it hard. Piplup also suffers from really bad learnset, so I wouldn't recommend it. Turtwig is also pretty meh, it isn't as bad as the previous two, but its typing and learnset aren't stellar. Though honestly, I think any grass starter that isn't Rowlet isn't a good choice.

... Mirror Move? Do you mean Mirror Coat?

When I battled her, Chandelure carried and 6-0ed with Flash Fire, so, my suggestion would be to use Arcanine or Houndoom with Flash Fire. Their damage outputs will still be quite high even through resistances. Hariyama/Persian with Fake Out also might help. Flygon is a valuable asset, especially with Rock Slide and Magcargo might help. You could also go hunting in Ametrine Mountain for a Horsea and evolve it with a Link Stone for Kingdra.

Showdown Name: Scyl Dark Template. Ace Trainer Female. Scizor -Bullet Punch -U-Turn -Roost -Knock Off Slowbro -Scald -Psyshock -Slack Off -Calm Mind Garchomp -Earthquake -Outrage -Rock Slide -Stealth Rock Togekiss -Air Slash -Thunder Wave -Heal Bell -Roost Chesnaught -Drain Punch -Leech Seed -Toxic -Spiky Shield Heliolisk -Thunderbolt -Volt Switch -Hyper Voice -Hidden Power Ice