As a general rule, kill mobs and complete all quests and you should not be outlevelled by mobs unless you venture into areas you are not prepared for. If you are level 5 (and not nearly 6 with just one or two enemies to go) and cannot find anything but level 6 to 8 enemies, you have almost certainly overlooked something, but what it is would be hard for us to guess.
This link goes to a beta-guide for Crysael that is still mostly current, so if you are desperate you can check it to see if there are quests you have overlooked:
http://guides.gamepressure.com/divinityoriginalsin/
