Scyl

... 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