Teaching spells is easy enough if you fight in the desert east of the opera house in the world of ruin, so focus on the main ones like the cure and life spells, Osmose, and some of the higher level black magic spells. At most you'd have to level secondary characters to the mid 30's. As long as they have decent spells at that point (and sometimes even if they don't) they can hold their own against pretty much anything with one or two of your main characters mixed in with them. Slapping the moogle charm on Mog can also get weaker characters through a dungeon without fighting when you're forced to use them. Even then though, the game itself isn't too hard anyway, and like I said, by the time you hit your mid 30's for levels most characters can hold their own, especially in regular battles so don't worry too much about leveling everyone. I don't know what it would take for characters to hold their own in the Dragon's Den in the GBA version though, as I've never gotten around to playing all the way through that version.