I was playing on Hard and definitely noticed that the only way to reach level 5 was to 100% all quests you could complete up to that point, explore around for additional XP in high-level areas where there are no enemies, and then either do the lighthouse fight (which has a level 5 boss) or engage some groups of level 5 enemies. So I do think that a few more level 3 and 4 enemies would be a good idea.
