Alright, thanks guys, it's a good thing I saved Mog. I'll probably beat the game this summer.
Without random encounters you only need to worry about bosses, but if you make a few runs through with your main party clearing the bosses then Mog party can eventually fly through without any trouble. It's a little more time consuming, but generally not as boring as leveling a third party.
Once I check the dungeon's layout again, that might make more sense. I thought everyone had strictly isolated routes?