How To Beat Magearna Consistently

Stored Power + Draining Kiss Magearna
Counters: Skeledirge, Clodsire, Heatran, Delphox, Gholdengo, Iron Moth, Volcarona, Tinkaton, Armarouge Yawn, Encore, Phazer, Trick, Slow Sleep
Unique Answers: Tera Steel Kingambit, Tera Steel Ting-Lu, Trick Magearna, Trick Gholdengo, Trick Slowking-Galar, Trick Rotom-W, Encore Magearna, Yawn Slowkings, Yawn Gastrodon, Yawn Torkoal
Notes: Tera Steel and Tera Fire are great against this Magearna set

Choice Specs
Counters: Magearna, Slowking-G, Slowking, Heatran, Amoonguss, Bulky Gholdengo
Hates Trick: Skeledirge, Blissey, Iron Moth, Toxapex

Shift Gear + Calm Mind
Counters:  Skeledirge, Iron Moth, Volcarona, Blissey, Heatran, Clodsire, Iron Treads, AV Magearna, Slowking-Galar, AV Amoonguss, Yawn, Trick, Encore, Phazer
More Answers: Gholdengo, Arcanine-Hisui, Volcanion, Cinderace

Magearna Lures
List: Tera Blast Ground Chien-Pao, Ground Tera Enamorus, Tinted Lens Braviary-Hisui, Ground/Fire Tera Regidrago

Pokemon That prevent it from setting it up
List: Dragonite, Baxcalibur, Enamourus, Sneasler, Landorus-T, Garchomp, Great Tusk, Iron Treads, Volcanion, Volcarnoa, Iron Moth, Cinderace, Arcanine-Hisui, Heatran
Skeledirge, Delphox, Ursaluna, Basculegion, Urshifu, Walking Wake, Roaring Moon, Samurott-Hisui, Thundurus, Zapdos-Galar, Ceruledge, Armarouge, Clodsire, Ting-Lu
Gholdengo, SD Iron Valiant, Lilligant-Hisui, Moltres, Torkoal, Azumarill, Zamazenta-C, Zamazenta, Quaquval, Sandy Shocks, Hydreigon, Slowking-Galar, Magearna, Kingambit
NP Tornadus-T, Braviary-Hisui, Regidrago, Scizor

Note: Not every mon listed in the prevent list 100% beats it, Magearna will force itself to tera to avoid an OHKO or the mon can only 2HKO Magearna at most.
It is best to make sure you have a check or two to beat it

Offensive Strategies and Notes on how to deal with Magearna easier
Tera Steel and Fire are the best way to Deal with Magearna defensively, they cover Magearna's most spammed moves really well.
Bumping your pokemon's speed to 252 speed is a good idea to prevent it from setting up; Great Tusk, Volcanion, and Landorus-T all appreciate this
Spikes are good at crippling Magearna in the long term
Volcarona and Iron Moth fit on a lot of Offensive teams, both mons have a good match up against Magearna
Pokemon that can check it after a +2 speed shift gear are worth considering such as Basculegion, Iron Valiant and Iron Moth