Why do I not remember this being quite so tricky?
Turns out there was some backtracking I had to do to get the left and right parties where they needed to be. The center party, the one I was using last time, had to go all the way back from where I had them at the end of the previous post to a switch at the very start of the big hallway they were on. That opened doors for the other two parties to go through and get to the other goddess statues from the Flying Island halfway through the game.
The party I had on the left, which had Mog in it, faced the statue monster called Doom. I mention having Mog in this party because Doom is weak against the Pearl element, which is essentially the Holy element from later games. I had Mog equipped with the Pearl lance as a weapon and the DragonBoots and Dragon Horn relics, which allowed him to do multiple Jump attacks and randomly use the Pearl spell. As a result, Doom was a real cakewalk by comparison.
The party I had on the right faced a boss simply called Goddess. I can't recall any particular weakness for this boss, but having Shadow in the party to use throwing stars on it. Goddess turned out to be the weakest of the three of these, because she came down the fastest.
Maybe I did these in the wrong order, I'm not sure, but anyway, I'm done with them now. I could have sworn one of the last two was supposed to be the hardest of them, but they were both pretty easy. Getting the middle party back to the save point they started from wasn't really that big a deal either.
From here, it's off to the final battle, but since I'm pretty sure that's going to be more than a little long, I'm going to save that for next time, so until then, stay safe, have fun, keep gaming, and DFTBA.