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I'm going to start this with my decision. Magearna: Ban.
Let's first talk about the most viable sets. The most common that I've seen is RegenVest. With boltbeam pivoting, it is very difficult to out-maneuver magearna. Its last 2 moves are usually dedicated to fleur cannon and focus blast/aura sphere, but flash cannon is also an option. It is very difficult to whittle down magearna with residual damage due to regenerator. Its 80/115 bulk with AV lets it easily shrug off most special attacks, like Gengar's shadow ball.
252 SpA Life Orb Sheer Force Gengar Shadow Ball vs. 252 HP / 4 SpD Assault Vest Magearna: 133-156 (36.5 - 42.8%) -- guaranteed 3HKO
252 SpA Choice Specs Alakazam Expanding Force (120 BP) vs. 252 HP / 4 SpD Assault Vest Magearna in Psychic Terrain: 116-137 (31.8 - 37.6%) -- 91.2% chance to 3HKO
252+ SpA Life Orb Sheer Force Chandelure Flamethrower vs. 252 HP / 156 SpD Assault Vest Magearna: 307-361 (84.3 - 99.1%) -- guaranteed 2HKO
Another viable set for magearna is it's old OU set, the stored power set.
Magearna @ Leftovers
Ability: Soul-Heart
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Iron Defense
- Calm Mind
- Draining Kiss
- Stored Power
This set is greatly powered up by a new ability. The best ones are triage and magic guard/bounce, but there are other niche options such as flash fire, levitate, and unaware. The user also has the choice of other items, like air balloon, occa berry, life orb, red card, big root, and some others. Triage lets it heal in a pinch, and draining kiss gives it a LOT of HP back. By buffing both defenses, and the guaranteed-crit urshifus banned, there aren't many ways to stop it once it's set up.
Those are the 2 best sets in my opinion, and I am not going to go into that much detail about any others, because I have found them to be less viable. Still, here is a list of sets that I've tried or seen, or just came up myself.
Solid rock wp + shift gear and sp
flashfire standard
levitate standard
Klutz trick AV
magic guard CM
Now that I've mentioned some sets, I'll talk about counters.
The most obvious weakness of magearna is magnet pull. Being a relatively slow steel type, it usually will get outsped before the AV set can volt switch out. The stored power set has a much smaller weakness to magpull, because it can stored power /draining kiss and do tons of damage. The 5 best magpullers I would say are Excadrill, Volcarona, Chandelure, Rotom heat, and alolan Marowak. All 5 of these pokemon can switch in (except sometimes excadrill) and proceed to deal massive damage. But you don't want to get too greedy setting up because it can either volt switch out, or just have flash fire and completely wall your volcarona.
What did I do to beat Magearna?
I used dig weavile.
Weavile @ Power Herb
Ability: Tough Claws
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Swords Dance
- Ice Shard
- Knock Off
- Dig
The idea was to bring it in to revenge kill noivern, swords dance, and then dig the magearna. It also came in handy against stuff like esurge magnezone.
I haven't even talked about how magearna has an amazing matchup against most of the meta. It beats gengar, noivern, weavile, corviknight, toxapex, chansey (with trick), and it even would beat banned stuff like pult and urshifu.
Summary: Magearna should be banned because it can beat most if not any counterplay with the right set.
But I think magearna is LESS of a problem than Gengar. Gengar is the next pokemon to suspect. It has 3 main sets: sheer force, no guard, and skill swap normalize. 2 of those outright beat any magearna set without bulletproof. I still want magearna banned, but we should definitely keep our eyes on that ghost.
, Volcarona
, Chandelure
, Rotom heat
, and alolan Marowak
. All 5 of these pokemon can switch in (except sometimes excadrill) and proceed to deal massive damage. But you don't want to get too greedy setting up because it can either volt switch out, or just have flash fire and completely wall your volcarona. 
. Gengar is the next pokemon to suspect. It has 3 main sets: sheer force, no guard, and skill swap normalize. 2 of those outright beat any magearna set without bulletproof. I still want magearna banned, but we should definitely keep our eyes on that ghost.
