Sounds to me like you haven't made it into Heavensward zones yet, so you're probably still a bit off from it. Kaiser Behemoth is a level 60 S rank hunt in Heavensward.

If you're looking for the normal level 50 Behemoth, in Coerthas Central Highlands, that is simply a FATE that spawns at random, with no trigger, you just have to wait for it.