I ran calcs uninvested Magearna and 252 HP Magearna vs every OU set, just to scope out what messing with the defense investment can let the set do
(some calcs in the set are a bit off because of lack of auto-weather):
The 1st green bar is where investment helps you avoid an OHKO, and the second is where investment lets you avoid a 2HKO.
and
Things that stand out: Lando's EQ, Tran's Magma Storm (not so much since you can't do much to it in return), Ash-Gren's Hydro, Specs Koko's tbolt, and Gengar's Shadow Ball. Specifically for this set, you seem like you want to be sent in, survive a hit while you're setting up a Calm Mind, then survive another hit while you pain split. If an attack does under 59.9%, we can eat at +0 and recover at +1. That range is:
Orange represents physical moves, which don't get affected by Calm Mind. So bulk only relevantly helps with this scenario versus Tapu Lele and Scarf Keldeo. Looking at how Special Defense EVs play into it, now calc'ing for 252 HP / 252+ SpD:
By tuning the special defense, we have the opportunity to add in LOrb Greninja, SpDef Tran, and Utility Tran into this. Tran is tough to beat around though, due to most of its sets running Taunt. Most grens won't bother staying in on a Magearna, so honestly not much worth investing in here.
Scoping out physical defense:
Taking Band Zygarde's Thousand Arrows is cool, as is messing with some Gliscor variants.
Fine tuning defense at this level can come down to preference, so let's work on baselines for offense and speed and see what we have left to work with for bulk. Offensively, it'd be nice to beat Pex with a +1 Thunderbolt into a +0 Thunderbolt. To do this, you need ~42.5% damage at +0 to get past black sludge recovery. Unfortunately, we can't hit this without going Modest or Electrium Z, so you'll have to hope you can play around with a well-timed Pain Split. 56 SpA gives you a great chance to OHKO defensive lando after rocks with Fairium Z, so we can roll with that.
https://www.smogon.com/forums/threads/sm-ou-speed-tiers.3587531/
Magearna can be tuned between 121 and 251 Speed, meaning we could have the set outspeed the following:
Adamant Tapu Bulu, Jolly Ttar, Timid Magnezone (Either Scarf or Sub so nothin much we can do here) |
(no longer requires Timid) 16 Spe Lando, Adamant Ttar, Uninvested Fini (Though roughly half run heavy speed investment according to the usage statistics), Adamant Mega Mawile
Looking at these, we could run 228 Speed EVs for 16 Spe Lando, 224 for Adamant TTar, 148 to creep the Band TTar set on the smogdex, 136 for Mega Mawile, or 100 to speed creep smogex Mega Mawile.
Doing 148 in speed gives you 52+ def leftover, enough to take 1 thousand arrows from band zygarde and taking an eq from Max Attack Lando, and jolly after rocks. Can't stunt on Gliscor, but most run Taunt or Swords Dance.