A duel opposes 2 players from the same nation (If the 2 players are from different nations, we should rather talk about PvP).
To duel someone, you only have to click his character then choose the 'duel' button. If he accepts, the duel begins.
When you kill someone, that person can choose to revenge without your agreement, so be careful.
Duels do not have any rules; however, you can choose to duel in the Arena by talking to NPC Musician Thomas In Map 2 then choosing "Duel".
Doing so will allow you to forbid pots usage, for example.
For duels it is normally assumed by everyone that red potions are not used in the duel, The auto attack feature and some times running around. (for PvP most people say anything goes, but being cheap is still frowned upon.)
The new update makes it so level 30+ is needed for duels but If you switch to hostile then peace again you can duel with people. (From what people say the person under level 30 must hit first for damage to happen)
There are stats that help in duels, for example strong against Warrior 5%. That makes your attacks versus all warriors 5% stronger, Also Chance to pierce, lets you ignore the enemies defence and restance (res stats). Those are two examples.
Stones also help in duels, here are some of the following. Note: The stone and stat work for both forms of the class, example Arahan and Partisan.
Spirit stone of Rebel;
+1- 7% Strong against Warrior.
+2- 12% Strong against Warrior.
+3- 17% Strong against Warrior.
+4- 25% Strong against Warrior.
+5- 35% strong against Warrior.
Spirit Stone of Sura;
+1- 7% Strong against Sura.
+2- 12% Strong against Sura.
+3- 17% Strong against Sura.
+4- 25% Strong against Sura.
+5- 35% strong against Sura.
Spirit Stone of Slayer;
+1- 7% Strong against Assassin.
+2- 12% Strong against Assassin.
+3- 17% Strong against Assassin.
+4- 25% Strong against Assassin.
+5- 35% strong against Assassin.
Spirit Stone of Mage;
+1- 7% Strong against Mage.
+2- 12% Strong against Mage.
+3- 17% Strong against Mage.
+4- 25% Strong against Mage.
+5- 35% strong against Mage.
Spirit Stone of Raid;
+1- 2% Chance to Pierce.
+2- 3% Chance to Pierce.
+3- 5% Chance to Pierce.
+4- 8% Chance to Pierce.
+5- 15% Chance to Pierce.
Spirit Stone of Fresh;
+1- 3% Skill Speed.
+2- 12% Skill Speed.
+3- 17% Skill Speed.
+4- 25% Skill Speed.
+5- 40% Skill Speed.