Isn't this a bit of a mismatch? Shedinja has one win condition, which it's apparently never used in-character which means it's relying on luck, and will die from any one hit. If it so much as scratches her and triggers Encounter, it dies. There's no timeframe given on the spirit stealing anyway, so assuming it's a single glance and it happens instantly is baseless. Not only that, but the Pokedex entries don't say "looking at it's back", they state that what triggers the ability is peering into it's body from behind. A single look during battle shouldn't be enough, or at least shouldn't be assumed to instakill.
If she uses Encounter or Ice Fire for a second it dies, and she won't be holding back nearly as much as she would normally given her opponent is a hollow floating shell whose mind she can't read due to it lacking a physical brain to send signals. If anything she'll asssume it's not a living thing and attack it, resorting to minuses when physical strikes don't work. How does any of that make Shedinja more likely to win? Not to mention any other damage it deals will be healed with AF, and even the effects of Spite could be recovered if necessary.
I won't vote since I still think this is too one-sided, but can anyone please provide a single instance of Shedinja turing around mid-battle to steal an opponent's soul? If there aren't any, then I'd be inclined to believe it won't be fighting with that as one of it's main strategies as some people seem to be arguing. "it turns arount" or "she sees it's back sometime while they're fighting" are either baseless, or fail to take into account how Medaka would approach this fight.
She wouldn't keep trying to fight physically if that failed, and given the last person she couldn't hurt was Iihiko, I doubt she'd be willing to give it a moment to attack. I won't deny that her seeing into it's back long enough to have her spirit taken mid-battle could happen, but arguing it's the most likely outcome, and would happen more often than Medaka using Ice Fire or Encounter for more than a second is an assertion I whole-heartedly disagree with. Unfollowing after this because I don't intend on debating, and I've said all I really wanted to.