FINAL FANTASY XII THE ZODIAC AGE

A note on how to spawn the Behemoth King since all guides written up to 2009 are wrong. This is copied from ffwiki however it is me who wrote it there and me over on gamefaqs who found this strategy to begin with:

This mark can be tedious to spawn, since all of the monsters in Ice Field of Clearsight, and the Giruveganus in Edge of Reason, must be cleared, including respawned enemies. The quickest way of doing this is by killing the Giruveganus in the Edge of Reason section first, then returning and killing the 29 enemies in the Ice Field of Clearsight and Behemoth King will spawn in place of the Giruveganus. The player does not need to kill any other enemies in Edge of Reason apart from the Giruveganus, or spawn the Mandragoras, to fight Behemoth King.

Also, if yoi reach trial mode level 90 you can beat behmoth king there and get the achievement(i did that for deathgaze) but you still need to do the hunt so you can spawn yiazmat.

The below is also taken from ffwiki, so credit goes to whoever wrote it:
Behemoth King is an Elite Mark in Final Fantasy XII. The hunt becomes available after the marks up to Fafnir have been vanquished.

Behemoth King can be found in the Edge of Reason area of the Feywood. 

Having a Ally:Any = Chronos Tear gambit may come to use against Slow and Stop. The player can also set up a Haste gambit, as Haste cancels Slow.

One strategy is to cast a Reverse/Decoy on one character and use the tanking teammates to pound it. A good tactic would be to use a Masamune/Kumbha + Genji Gloves combo. The player can keep the King in its magick-immune phase by always casting Arise twice on it after it becomes immune to physicals.

The player will be hard-pressed to heal, as the Behemoth's Kings physical blows can do several thousand damage even with Protectin effect. Its various elemental spells can be guarded against with the right equipment, and the player can use the spell Shock for offense when it is immune to physicals, as it animates quickly and does a lot of damage.

When it is in low health, a Quickening chain, if the player can do one long enough, can kill it quickly.