Yiazmat may have 50 million+ health points but Omega mark requires you entire party to be maxed out before even attempting to get your ass kicked.
The thing is, is that niether of these bosses is hard, just time consuming. With the right gambits of reverse and decoy set up, you can easily topple every monster in the game. The problem is that, outside of espers and concurrences, the most damage you can do is 9,999. If we round that up to 10,000, it would take you roughly 1,100+ hits to take down Omega's 11 million HP. That's well over 5,000 hits for Yiazmat. After spending hours on Omega and then more time on the Hell Wyrm, I just lost interest in fighting anymore.
Zodiark is a waste, just like every other esper in the game. If you trigger his final attack, he does 60,000, which isn't worth using all your MP, in my opinion. In fact I went though the whole game without using the espers. They are a waste of your MP and rarely bring much to the battle that your own characters can't do better.