southern part of golmore jungle is a good place to level up you might need to advance the story a tiny bit to go south buts its a good place to level up. the enemies can be kinda tuff at that level though.

as for the esper in the Zertinen caverns your probably better off adding another 20 levels to your team and going in with long ranged weapons. i was on lvl 30 somthing when i tried to beat it and i lost im on 40 now but i havent gone back and tried yet but right now im going on hunts then im going for the esper.