The set I use:
Magearna-Original @ Assault Vest
Ability: Soul-Heart
Tera Type: Ground
EVs: 248 HP / 68 Def / 192 SpD
Calm Nature
- Fleur Cannon
- Ice Beam
- Volt Switch
- Tera Blast
I am of the opinion that Magearna has an actual place in Nat Dex Ubers, a niche place, but a place nothing less. So what does this "place" have to with anything, the AV set is what I'm gonna mainly talk about. The added special defense means that it can sponge special hits quite well, but nothing more important than being an excellent check to the incredibly centralizing Yveltal. Since both of Yveltal STABs are resisted, plus the fact that Magearna can hit Yveltal for Stab super effective damage with Fleur Cannon, which even without investment, will always OHKO Life orb Yveltal after the Orbs damage.
252 SpA Life Orb Dark Aura Yveltal Dark Pulse vs. 248 HP / 192+ SpD Assault Vest Magearna: 52-62 (14.3 - 17%) -- possible 6HKO
0 SpA Magearna Fleur Cannon vs. 0 HP / 0- SpD Yveltal: 398-470 (101.2 - 119.5%) -- guaranteed OHKO
This doesn't just apply to just Yveltal, with it's Zacian-Crowned shared typing, it can also stonewall Deoxys-Attack, thanks to it's expanding Force resist, and solid bulk accompanied by Assault Vest, and partial fairy typing making it neutral to Low Kick, along with easily switching into special moves that aren't Fire or ground type, or Primal Kyogre.
Now Magearna has alot of flaws holding it back from ever being anything above a C+, like a horrible Primal Groudon match, being setup fodder for Zacian Crowned, unimpressive special attack especially without investment, the Ho-oh issue, and lack of reliable recovery.
However, these can somewhat be undone with careful teambuilding, like Alomomola wish passing as an example, so Magearna can last longer in longer battles.
Anyways: https://replay.pokemonshowdown.com/gen9nationaldexubers-2499243966-c6mkc3xh97y5knj2au9l009u0154foupw Magearna's special hit taking abilities at full effect.
https://replay.pokemonshowdown.com/gen9nationaldexubers-2499251650 Not too much, but can prob show it's capabilities against stall quite decently thanks to volt switch.