Hmm, I would say the islands closest to Heaven/Hell, but it doesn't appear you have the right aircraft yet...

Try fighting some enemies on the Centra continent, especially in the forests. As you're on the World Map, you can save regularly, but stock up on Tents beforehand, so you don't use so many Potions.

Also, junction mid or High level magic (-ara or -aga magic) to you Str stat, if you have it. If not, distribute your GFs so at least three have this ability. Failing that, junction to your HP stat.