There's a reason none of the situations you mentioned exist in single player games. I'm not talking about SRPGs, though.

I suppose I should say the only reasonable way to make an RPG appear difficult is to rely on design crutches. If you implemented true difficulty into an RPG, you'd get what would essentially amount to a multiplayer component with "bots." That'd be fine, I guess, but in single player form? Do we really want to force people to overcome extremely smart AI in a perfectly balanced environment just to progress through a game's story?

But let's take a look at Yiazmat in comparison to other FF superbosses:

Yiazmat's AoE wind attack is easily countered by equipping wind breakers. Death touch isn't a big problem if you have Arise gambits, though it does get pretty brutal at the <10% mark. You don't have to fight him all at once, as you can go to the save point, save, and his HP total will remain even after you turn the game off.

Omega Weapon has a set turn he uses Terra Break on, and Defend takes care of it. Not much else he uses is a threat.

Emerald's Aire Tam Storm or whatever is countered by equipping only necessary materia. His physical attacks are incredibly strong, though -- Emerald is very tricky to beat without resorting to KoTR miming... or W-Iteming 99 megalixirs.

Ruby I don't have much experience with, but I'm pretty sure he's also "cheaper" than Yiazmat with his Ultima KoTR counter.

Ozma's Curse can be... almost countered by equipping a whole bunch of status immunities. That won't save you from it oneshotting your entire party at times, though. Then there's Meteor, and the fact that it can cure itself... I would say Ozma is more "cheap" than Yiazmat is... by far, actually.

If I had to rank them on "cheapness," I'd probably go Ozma>Ruby>Emerald>Yiazmat>Omega. However, if you're looking for a quick but intense fight, you probably want Omega mk.XII or whatever it's called, in the Great Crystal. I would argue that he's the real superboss of the game. I'd even say you should get an achievement for beating him. I have now segued back to the original topic of this thread.