I'm glad too see other people have been having problems with the Behemoth Kings! The moment I got to the Archylte Steppe I fought one & its Heave attack bitch slapped me so I got a bit obsessive with character grinding since. I've managed to beat a couple of them by ambushing them so they get staggered instantly but apart from that they still beat me when they transform although I can put up a good fight! Has anyone managed to beat one of those gigantic enemies roaming the steppe yet?
As far as level grinding at this point (Chapter 11) I think it's a good idea! A couple of hours grind has taken the edge off the difficult just a tad!
Also (SPOILER)the beginning of Mah'habara, the Juggernaut fight is that totally optional because even with my level grinding he gave me a serious ass kicking!