I thought Jenova BIRTH's Stop was more like 90%, rendering damage problems moot. Hmm. Makes the fight more interesting but even if we assume 60% Stop, that's still kinda rough for Wakka, since Stop can just be reapplied, right?
FF7 BMG explicity says 60%, which fits my memory well enough (I recall Meeple calling it very accurate but Tonfa calling it terrible, so the range of experience suggests 60% is possible!). No idea if Stop can be reapplied or if it auto-misses against an already statused target, but since it can only be used three times (and one of those three will miss) I don't think it overly matters. And yeah I forgot about Osmose, which can only make things worse, though scaling arguments will vary there.
Re Mia... oh wow, I completely missed Ice Wall's existance. For some reason I was thinking Ice Lance was her best. Yeah, in that case, Mia equips the Wind Cane after all, and Ice Wall still breaks the distortion, so she wins. Edits away!
Do note that the stat topic figures for Mia assume the Dragon Cane already, though, so 190 is in fact her absolute peak damage for ice. Not that it matters.
EDIT: Oh, huh, I didn't think Retinite's defences started at maximum, but this must be me misremembering since Fenrir explicitly notes otherwise. Okay, my bad there, he does indeed spoil physicals pretty nicely in Godlike then.
Regarding Tail Spin, while it is not "magic" plotwise, it is magic in every sense gameplaywise, and does hit Spekkio, the same way Laser Spin does, along with hitting MDef. Chrono Trigger has exactly one other non-elemental PC attack spell, Poyozo Dance, which also works fine. Retinite simply resists all elements, he does not have MDef, so hyping him as doing anything but dying to NE magic makes little sense to me. We have no problem saying the same thing happens to Magus, and Magus
can't even face non-elemental magic in-game.
"Shadow" doesn't feel non-elemental at all... it's probably the most resisted element in the game. It is sort of catch-all for including lasers I guess (as well as Antipode spells), but it's certainly not non-elemental. In addition to the PC non-elemental spells, Lavos has several, such as Doors of Doom, Shadow Doom Blaze (neither shadow nor blaze), Obstacle, and Dreamless. A few other enemies have non-elemental magic too, though it's not terribly common.
Idly: You could run from Retinite? Really? That doesn't sound right to me but it could be something I'm unaware of. You can see him and avoid engaging him in combat, but once you've seen how he fights I'm pretty sure you're in a battle to the death. Could be wrong!