Prior to the mobile version, inflicting Berserk stops them from countering, making them trivial to defeat. The same effect can be achieved in any version with control, though the player can only use magic attacks then.
King Behemoths are only immune to the "Mini" status effect. As a result, they are fairly easy to crowd control through spells. Black Mages can attempt to Toad them, Blue Mages can attempt to Pond's Chorus or Doom them, and Oracles (for the GBA/mobile/Steam releases) can outright kill them easily with "Doom." Mystic Knights can apply Break to petrify them.
Another viable, albeit luck-based strategy, is to use Gaia, which has a chance of reducing King Behemoth's HP to single digits with Twister. Though magical in nature, King Behemoths will only counter Gaia skills with physical attacks.
In the GBA and following versions, Lenna's elemental Finisher ability will instantly kill it.
[view·edit·purge]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."