I found a Fiesta guide about the simplest ways to level, this will give you a short, simple, yet very helpful guide to lvling from 1 to 59, as fast as possible, without skipping anything and without losing anything. Of course, maybe this guide miss something. If you know, please feel free to point them out.

Levels 1-10

Once you hit level 5, do a Slime KQ. Youll instantly be about lvl 7 or 8, maybe even 9. From there, get fiesta gold all your quests, and do as many as fast as you can. Bundle em up - In other words, go out, do 10-20 quests, then turn em in at once.

Levels 11-20

These levels are best done hard and fast. If you have a higher lvl friend, ask them to help. Or better yet: join an academy. Get loads of quests (20/20) and ask them to quickly come kill all of your "kill quests". Same basic principle applies: Bundle your quests.

When you hit 20, get fiesta gold some t1 scrolls, and try to get a buff from the guild you are now in the academy of, and do the class change quest. It should be relatively easy.

Levels 20-30

These are best done in the lv 20 abyss up to about lv 28. Preferably in Duos. Mage / Cleric is a good duo, and archer/ cleric is too. Fighters can solo well enough. Remember to mob and aoe. This is your aoe training, sort of. If you get to about 27-29 and are tired of abyss, do some quick quests, but save some of them for turning in all at once. This can easily pop you from 26 or 27 into 29 or 30, and into the next level group.

Levels 30-40

Once you hit 30, do all of your remaining lv 20-29 quests. You should have been picking them up as you lvld. They should easily get fiesta gold you to at least 32. When you hit 32, do your 30-32 quests to hit 33. When you hit 32 or 33, there is a quest from the Title Merchant. Do it. The second part is a lvling quest (purging of moonlit tomb), that will lvl you completely. My advice here though, is make sure youre at 0% when you turn in the quest, because it gives you 99% of the level, and once you level, you cannot pick it back up. So get to 99%, pick it back up, and go complete it again. By killing the stuff to do it, you will level again. Turn it in to level yet once more!