After ending the current storyline you can fight the wolf queen again and she will give you about a level or so. Otherwise, there are two other ways to gain xp after completing the story - killing enemies and buying xp potions from the elf girl's shop. Although there are 5 stages currently, the story should cover about only 2 (basically around where you were) of them if you don't grind extra, or speedrun.
I know this is not really intuitive and I am working on mora and more story, but it takes quite a lot of time to make compared to stages. In the next update there will be more shops and ways to level up so that should help when it comes.