The first time I beat Yiazmat, I didn't retreat once. My party was all level 99, all licenses, and I was stocked with as many items as I could possibly have (well, except maybe elixirs/megalixirs). It's pretty much a matter of grinding out those first 30-40 life bars at first, since his attacks cause instant KO sometimes, which is very annoying.

When reaching the last 10 lifebars, I take the reverse+lure route with a tank. The thing is, his combo rate goes waaaayyyyyyy up after he uses his magical level-doubling move. The tank actually goes BEHIND Yiazmat, because when Yiazmat attacks someone in front of him, he attacks very quickly in succession, whereas when he's behind Yiazmat? He's gotta use that tail, and though he still has a high combo rate, the attacks come more slowly. This means fewer chances to cause instant death.

It took me seven hours to win. And I didn't even know he was weak against Dark at the time. I had pretty much every weapon, though not the Yagyu Darkblade. I used the Tournesol, the Ultima Weapon, anything strong and non-elemental. I had 21 Chronos Tears left...and around that many Softs too.