Sweeper Soul-Heart Magearna:
Magearna @ Wise Glasses
4 HP / 126 Def / 252 SpA / 126 SpD
-Fleur Cannon (the SpA drop is crappy, but you'll most likely OHKO your opponent with this, raising your Special Attack by one with Soul-Heart, and this makes it more bearable. With 244.5 base power (STAB + Wise Glasses), only one Special Attack drop will be acceptable, and you can OHKO your opponents before using it. If you OHKO your opponent, it will be brought back to 394.
-Shift Gear (I know the Attack increase won't be anything since you are full Special, but anyways, the 2x Speed increase wil be great, since you already have great Special Attack. After one boost, Magearna will become a terrifying sweeper).
-Mirror Shot (most will ask "why Mirror Shot when she has access to Flash Cannon? Well, the accuracy drop is VERY useful, and Magearna already has a great Special Attack, while after the modifiers it will have 106.75 base power, which with an accuracy drop is EXCELLENT.
-Aura Sphere (covers 5 different types, one of them being Steel, which resist both of Magearna's STAB. AND it always hits. 88 base power after the wise glasses makes it AWESOME, and Fighting is always a good type to have a move of when your Pokemon is a Fairy type. Aura Sphere does the job well, and at least deals a normal effective damage to one of her 2 weaknesses, which is Fire. So, is just a strong coverage move (but is always nice to have coverage).
I've been swept by this in a random Showdown! battle before. The opponent killed me with just Magearna, they didn't even have to use another one of it's Pokémon. It actually IS OverPowered, but I pretty much hope it does not get a nerf, since is one of my favorite ever Legendary Pokemon (I classify Legendaries, Mythicals and Ultra Beasts all as Legendaries, as 20% of the world).