The Behemoth King is an optional boss in Crisis Core -Final Fantasy VII-. It is encountered near the end of the game, in the area after Zack goes down the hole when they revisit Banora Village. To encounter Behemoth King, Zack must first find the Goddess Keys in Cage of Binding to enable him to access Howling Fang.
Start off with casting Wall and Regen on Zack. Try to position him behind the Behemoth and land some critical hits on it. Due to the small landscape, positioning Zack might be a little bit difficult. Use Tri-Thundaga or something of the like when possible.
Behemoth King will most likely use its Gyro Tail attack to counter Zack, and sometimes it will cast Quake or attack with the powerful Comet Blast. These attacks are unavoidable, but should not be too much problem if Zack has high Spirit stats.
Wall or Regen is not necessary, and if you have not obtained Tri-Thundaga, it is still possible to defeat. Spam Graviga seven or eight times to get the Behemoth King's health low enough so a regular sword slash can kill it.
Behemoth is a beast mentioned in Job 40:15–24. In addition to mythological creatures, it is likened to elephants, hippos, rhinos, and bison. Metaphorically, the term behemoth denotes "an extremely large or powerful entity."