Well, this is the best way to level up in Conquer Online, I don't think I missed one, if I missed one please report it to me.
Screenshots are made with few characters of mine called: VitaLitY, maRtha, Adaemus, ~AllKaDeaS~.
There are some quests where you can do faster with one kind of reborn character, and it's simply to use, that why I used 4 characters.

1. Mentor:
As a Mentor, you can claim contribution from your apprentices. When apprentices kill monsters, get quest completion experience or do online training, the mentor also gets experience.
This is one method to level fast, when your apprentices level up, automatically you will get some ExpBalls, and you can claim when you want. (If you have something to claim)
My advice it is to add at mentor list, the person who own archer or warrior, because they level up faster than other classes.
When you reach level 70-109 you can add 2 apprentices,110-119 you can add 3 apprentices,120-129 you can add 5 apprentices,130-133 you can add 7 apprentices, after 134 you add 10 apprentices.

If you have more apprentices, you level up faster.

2. Online Training Ground:
You may level up at the training grounds if you are level 20 or above. Here you'll obtain experience by attacking stakes or dummies. It won't wear out your weapons, or cost you any life or mana.

There are training ground teleport NPCs named "Boxers" in Twin City, Phoenix Castle, Desert City, Ape Mountain and Bird Island, and to join in Training Ground you need 1,000 Silvers.
Once you guild occupied the guild arena you won't pay nothing for join in.

3. DisCity:
Hundreds of years ago, in the decisive battle between human and demons, Ultimate Pluto, the leader of the demons, was beaten by the forces of humanity. However, he escaped and swore an oath to eternally seek vengeance upon those who had disrupted his plans.

This quest it is only for players who reach level 110.
What you have to do:
- If you enter the city, you will get experience worth of half EXPball. After that, kill Phantoms, Undead Soldiers, Undead Spearmen, Revenants and Eidolons to gather five Soul Stones. Give them to Solar Saint, he will send you into the Hell Hall. Only 60 persons can pass this level.
While you kill monster they will drop meteors, Dragon Balls, gems, and lot of gold, but while you kill them you can get experience too.

After you pass first room, you'll be awarded with experience worth 1 EXPball.
In this room you must have Strength and Strategy to pass it easy.
- If you pass first room, now you are in second room that is called Hell Hall.
This room is 100% Strength room, because you must kill monster until a message will show up that you done.
Trojan must kill 800 monsters. Water Taoist must kill 600 monsters. Fire Taoist must kill 1000 monsters. Warrior must kill 900 monsters. Ninja must kill 800 monsters. Archer must kill 1300 monsters.