I don't think I ever seriously took time to level up. Although I spent time around mideel trying to gain money to buy food for my chocobos to get KotR (maybe this was my leveling up). I never enjoyed fighting in the Sunken Gelnika, the monsters were annoying, they usually put some kind of effect that cause serious problems in trying to defeat them.
Like everyone else I recomend Mideel, if not the Nothern Crater.