@SanitaryCockroach It would be pretty weird if he can only level up by killing humans from now on. Maybe the wolves simply didn't qualify anymore. He would need to slay more dangerous monsters.
@musicfreak12 To breaktrhough lvl 15 cap, they need to defeat higher ranked monster with a party with the max member of 5 people (or 4 people? I don't remember). Lvl. 15 is a cap that many adventurer have because not many people got the balls to defeat higher ranked monster.