While dueling often has a specific meaning a Dueler is technically any player that doesn't quick trigger so much as fight one on one battles of hand to eye coordination and speed. This is often the role of some slower players, though this is a disadvantage should you play man. If you prefer confrontations, don't like ghosting, then this is probably the job for you. Duelers are the front line, and many games are won or lost based on their actions.
In a duel a player makes it as hard as possible to get hit by his opponent. He does so by standing sideways (shoulder facing his opponent) and blocking his shoulder sensor. By standing sideways he reduces the chance of attack on his front and back sensors. The goal is to make it impossible for him to reach your front or your back. While still keeping your shoulder and gun sensors protected.
Of course you still need to attack, it is advisable to aim at your opponents gun while still blocking your shoulder sensors, and not getting shot in your own gun. Easier said then done. One can avoid many dueling loses simply by not putting your gun in the direct path of a beam of laser. How fast one shoots is definitely an advantage here.
The gun is the preferred target because it is almost guaranteed
to be open, it is often more sensitive then the notoriously shoulder
sensors. Plus a single shot down the barrel of the gun often removes two
shields rather then just one, because it has a sensor on both sides of the
barrel.