Final Fantasy XII: Revenant Wings

First of all, it's Yiazmat.
Second, look on the board first.

Yiazmat, the boss of Midlight's Deep, is FIERCE! He has the single greatest amount of HP in the game, the most devastating attack power, and lots of Yarhi for support. The best strategy to kill Yiazmat is to isolate him from the other Yarhi and to focus on using every single offensive unit as possible. The best party to use in my opinion is Vaan, Penelo, Kytes, Filo, and Llyud with the strongest lvl 3 Yarhi you have (i.e. Bahamut), a healing unite, two ranged units, and a flying type unit.

Espers:
I: Any
I: Any
II: Carbuncle (HUGE MUST)
II: Golem
III: Gilgamesh (if you don't have him any strong melee esper is fine)

Leaders:
Vaan, Penelo, Kytes, Basch/Ashe (whatever suits you best), Llyud

I suggest you go first work your way to the summoning gate either clockwise or counter-clockwise. Dismiss all of your rank I espers. Now, summon Gilgamesh to Basch/Ashe. Max summon Golems onto Vaan and Kytes because of their quickenings. If you still have Affinity, summon some more Carbuncles onto Penelo for more healing. If you think you need to, spend a bit of time fighting enemies to build up Quickenings and to fortify your espers. When you're all good to go, have Kytes kill the Carbuncles surrounding Yiazmat. Send Vaan and every golem to attack Yiazmat. Everyone else should just provide support and kill off the enemy espers. If the enemy espers are troubling you, send back a few golems.

Actualy i do the same thing they do. I'm on my second time through midlight's deep to get the anastasia high enough before i get deadlyboredlike. oh! Curlyhairboy has his anastasia's attack at over 500!!! go go go!!!
anyway i was thinking. whats the fastest way? i was thinking about rushing him head on right when i start. it gets really really annoying if you die after you spent so much time going around to the summoning gate. God those anastasias are godly. some pplz cant even see vaans arms anymore. just a big yellow line on the enemy he's killing. its insane. there's on youtube. look up "final fantasy XII revenant wings anastasia". He's solo killing yiazmat. It's awesomeawesomeawesome. And its seroiusly fast. Theres no limit so its awesome.

Vaan Solo with Anastasia blade... Oh wait, you aren't that strong yet. XD

I suggest a team of Vaan, Basch and Penelo with the others, you can use anyone (Filo is preferred even though she is flying.)

Golem and Carbuncle are musts but for level 3, I suggest either Leviathan, gilgamesh or Shemhazai. However, if you summon the physical espers under Filo, then golems don't matter and you can use Cuchulain or Balasa instead since Filo can nullfiy the stop effect.
Leviathan is good because it is great at soaking damage and if summoned under Filo, it can offset its weakness to stop.
Gilgamesh deals huge damage to ranged foes.
Shemhazai is a VERY strong esper due to her high speed to attack and she easily soaks up the damage that Yiazmat deals.

You should have easily been able to defeat the other espers at this point. The important thing is to make sure you are ready for Yiazmat. Have Filo or Vaan rush in first and use all the stat debilitating moves (steal and trap) on Yiazmat before immediately pulling them back to engage it with your main army. Keep Penelo and her carbuncles to the side so that they don't get hit by the breath attack. The flow of Cuchulain and golem are annoying but if you have a Leviathan and Shemhazai out, it can easily deal with them. The 4 carbuncles are important. Kill every one EXCEPT the one you know you will not hit because if they all die, EVERY SINGLE ENEMY ON THE FIELD ATTACKS YOU. However, if you focus everything on Yiazmat it should die in about 2 minutes.

I just eliminated all the other Yarhi and then attacked like crazy... Something got wierd and he was attacking the wrong way (off to the side) so I just won =D

My party was Vaan 54. Penelo 53, Llyud 55, Balthier 53, and Basch 55
Espers were White Hare, Djinn, Wyvern, Lamia, and Exodus because of the AoE attack of the Wyvern, the confusion from the Lamia, and Exodus because his elemental advantage over Yizemat (alos he has meteor)

Basicly i just fought my way through the espers to the summoning gate. Basch's Shining Ray (aquired at 51) is AMAZING it 1 shots most of the espers on the map except for the golem and Yizemat of course and causes AoE damage as well. After fighting through i just opened the fight with Vaan's quickening and just threw everything i had at him