>>33572335(copy and pasted from john smogon himself)
Checks and Counters
Steel-types: Heatran, Ferrothorn, specially defensive Excadrill, Celesteela, Mega Mawile, and Scizor force out Magearna, as they can tank a hit and KO it back. However, a Z-Move such as All-Out Pummeling or Gigavolt Havoc, or a coverage move such as Hidden Power Fire in Scizor's case, can invalidate these Pokemon. Jirachi can effectively defeat every variant of Magearna outside of the rare Never-Ending Nightmare set. Assault Vest Magearna defeats offensive Magearna, but it can lose to sets with Corkscrew Crash.
Fire-types: Fire-types such as Volcarona, Mega Charizard Y, and Heatran are solid checks to Magearna by typing but must watch out for specific Z-Moves that can invalidate them. Alolan Marowak is a solid check to Magearna in general, as it blocks Volt Switch due to Lightning Rod. However, it once again has the same problems as many other "checks" because it drops to Twinkle Tackle after Stealth Rock.
Ground-types: Landorus-T, Garchomp, and Zygarde force out Magearna with their powerful STAB moves. They also make Magearna users think twice before mindlessly clicking Volt Switch. However, Landorus-T, Garchomp, and Zygarde are not guaranteed switch-ins, as they will be destroyed by Fleur Cannon and can be lured in and KOed with Shuca Berry sets.
Specially Defensive Pokemon: Amoonguss, Toxapex, Zapdos, Venusaur, Chansey, defensive Volcarona, and Assault Vest Tangrowth are some specially defensive Pokemon that can wall Magearna depending on its Z-Move. All of them make fine checks to Assault Vest sets.
