I managed to last-second pull two Winter Tharjas, but I'm not sure which IVs would be more consistently useful. One is +Res -Spd (which would make her my best magical tank at max level afaik), and the other is +Atk -Res. I could probably give the -Res a distant def seal and compensate for the Res loss but, I'm still just trying to figure out the ideal way to build her tbh. Any ideas would be grand! o/
Depends on how you want to use her. Choosing the spd bane drops her out of the 165-169 BST tier to a lower one, which would slightly hurt her arena score.
With that said, spd is probably her best bane, because it's her most useless stat, so if you are not going to use her in your arena core or you don't care about your score too much, go for -spd. If you want her to be a part of your arena core, choose -res.
+Res and +Atk are both excellent boons but -Spd is her perfect bane so I'd be inclined towards that as it makes her tankier and she can rely on Vengful Fighter and specials to take down everything that come near her. Rauðrblade+ is also fun to boost her attack and she can run Aether to stay alive, mine has survived Reinhardt and killed him on return in Grand Conquests.
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