With the combinations of the power, bulk, ability,, and versitality, Magearna is a potent threat in the metagame. It can run sets like Shift Gear, Assault Vest, Offensive Trick Room, and Choice Specs.
Shift Gear Magearna is really good because of its ability to clean through teams with Soul Heart + a Z move. It can run many sets on Shift Gear. The Z moves it can viably run are fighitnium z, steelium z, fairum z, ghostium z and electrium z. It has coverage in Fleur Cannon, Thunderbolt, flash cannon, aura sphere, Focus Blast, shadow ball, ice beam. It also does not have a hard time setting up because of its brilliant natural bulk. WIth a timid nature, it can outspeed scarfers like Garchomp. With a shuca berry + ice beam, magearna can lure in zygarde, Landorus-T, and Garchomp.
Assault Vest Magearna is a wonderful pivot with a great special defense stat. It is a tank. The point of this is to deliver slow volt switches and check pokemon like Protean Greninja, Latios, Weavile. It can also check Tapu Koko and keldeo to an extent and can deliver massive damage. It also gains momentm against stall, unless the stall player brings in dugtrio on a volt switch. It is a really nice check to non fightinium z bulu variants.
Offensive Trick Room Magearna is extremely similar to trick room, except that it is outsped by slow mons, has limited turns to abuse, and can afford to run max hp. It can help teammates like Alolan Marowak.