You can level up fast by completing all the assigned tasks and challenges. Also, interacting frequently with other characters and joining clubs can help.
One way is to focus on completing quests. Quests usually give a good amount of experience points. Another is to battle strong monsters. Although they are tough, they offer more XP when defeated.
Often, loose ends like unanswered questions about a plot twist or the fate of a secondary character are neatly tied up. For example, finding out why a particular event happened or what became of a character who disappeared earlier in the story.
One way is through clear and concise explanations. For example, if there was a mystery about a character's past, the author can simply have the character tell their story in a conversation. This gets the point across quickly.