[quote=@Cleopatra]
[@DarkwolfX37]

It's more or less the idea of taking in the soul gems despair into her own, or otherwise directly removing it. Perhaps you could go down a psychologist route and lower despair through emotion and the like(More effective pep talks because super psychology), but that somewhat overlaps with our medicine specialist magical girl. (Though they're more about using medicine then psychology to lower despair I suppose)
[/quote]

Maybe I'm thinking of it wrongly but aren't the soul gem's corruption, what actually turns the girl into a witch, and despair, the mental afflictions and general concept, different things? As I remember it, which could be completely wrong, the corruption brings about despair because it's literally their soul being darkened, and despair increases the rate of corruption because it acts as a sort of breeding ground for darkness in their soul.
Considering them to be literally the same, I completely agree that the wish couldn't work. I just wasn't thinking of it that way before, and so I'm hoping to clarify before I decide on a replacement wish.