You don't need to make a lot of levels, just enough to combat "normal" enemies. Maps are filled in some area with very strong enemies.
Usually you can advance normally (or easily) when monsters are the same level as your group.
If they are bellow, they are "black" when you target them.
I recommend to always print the monster's status. go to settings, and enable the "always show".
It will show above all enemies :
Difficulty:
Red : very hard (5-10 more level than you )
Yellow : hard
White : normal, but still little hard. Give the best gameplay experience and fun battles. (2-3 levels more than you)
Blue : fun and easy (same level than you)
Black : Too easy, these enemies don't auto-attack you at all, even if they are aggressive. (lower level than you)
Aggressivity:
Dot (•) : not aggressive
Eye (|) : aggressive on sight
Radar (((• : aggressive if you run near them (always walk), and if battle are making noise (usually when an enemy die and produce noise, you see sound waves)
Ether {X} : Detect Ether skill usage during a fight.
Friendliness:
If there are dots around the aggressivity (not an english word?) notification, like ---(•)--- then it will join battles if you attack enemy of the same species.
So if you see a red ---(•)--- don't worry, you can go near, it won't attack.
If you see a red (|), then don't stay in the area, you need to fight monsters in another place to advance in the story
PS:
Can someone answer my post in the "hype thread" about missions? thank you
Edit:
Ah yes, ether detection, thanks Shadow.