Don't be afraid to return to previous levels as doing so gives you the opportunity to unlock new Scene Cards which you can use to make your characters more powerful and make it easier to advance.
Free Bonus
If you log in every day you will receive a free bonus which you can slowly build up to a substantial level.
Medals
When you complete quests you will earn medals whcih correlates to a different hero. Make sure you use these medals whenever possible to level up the characters.
Scene Cards
Scene cards increase your stats and give your characters special abilities so it is important that you regularly check the relevant sections for them and use any you have obtained on leveling up characters. Doing this will make a big difference during quests.
Skills
If you have two Scene Cards that have the same skill you can fuse them together to upgrade them so they get more abilities.
Synergy Bonus
You will receive a synergy bonus when you equip a character with Scene Cards of the same typing, a synergy bonus will boost your characters stats. If the Scene Card has SK next to it it indicates that your current character will also receive a skill.