Behemoth King is...kinda hard, but only when your characters are like level 65-70.

I've found my leveling spot in the Henne Mines unlocked phases, I'm sure everyone knows the easy leveling you can do on the Undead Bats there.
I'm not planning on completing the Bestiary, but might be tempted, I don't know. My goal is to kill off Yiazmat, Hell Wyrm, get all espers, kill Omega Mark XII and too get some of the ultimate weapons. After I done all that, then I'll complete the bestiary and complete the game.