RPG Insanity

Where is the last unknown location(it's between Lixey\inland sea and the battle arena in the enemy guide)? Do I have to beat grams on all four difficulties to reveal it? If I do then I'm screwed because I can't beat her on hard and expert. And are the last four chests in that location? (I have 536\540)

"Deep Sea" with 4 enemies comes between "Lixey" and  "Battle Arena" and should be available to you.

Where is the Deep Sea location on the map?

There, found it! Must have airship first!