How to defeat Yiazmat with zero frustration on a normal run. You can technically make the fight even faster, but this setup allows for minimal intervention.
1. Setup gambits as shown with revival magicks or Phoenix Downs, Stop recovery with Chronos Tears, Esuna, and buffs (especially Shellga). For the main DPSer (preferably Shikari with Yagyu Darkblade), just keep him permanently berserked.
2. Apply the following breaks: Wither and Expose. After applying Wither until it starts missing, you can safely turn off Curaja and Bubble gambits if you feel comfortable. You can also apply Shear if you have a Monk since poles are resisted by Magick Resist instead of Defense.
3. Stop applying the breaks and sit back and relax as your characters mow down Yiazmat.
Shell is an important buff since it reduces the chance that Yiazmat applies instant KO. You don't really need to use Decoy, especially since you'll fail to cast Decoy a lot with Shell on your characters. Other useful buffs are Bravery and Haste. I chose to Berserk only one character to keep things more automatic; two non-berserked characters can revive each other and the berserked character should one fall.
You can speed things up further by switching revival gambits to whatever gives the least HP (either Phoenix Down or Revive depending on what your Phoenix Lores or Magick Power is) to keep characters at critical HP more often, which I didn't do here.
Vaan: Shikari/Red Mage (need Exodus for Dragon Helm) - Main DPS (Yagyu Darkblade+Dragon Helm+Black Robes+Germinas Boots)
Fran: Monk/Time Battlemage - Buff support, Breaks (need Zeromus for Shear if you want)
Balthier: White Mage/Machinist - Buff support, Secondary DPS (Mithuna+Dark Shot+Black Robes)