When it comes to dueling, you will, of course, never "win." Okay, one time...
However, you will annoy everyone.
If dueling a sorc, zon, or ranged character, leap close to them, and try to get them in a stun lock of sorts, by making a series of shorts leaps around them, or forcing them against a wall.
When dueling a melee character, stay out of range of their weapons. Avoid leaping OVER them (for some reason, this doesn't work well, and they will kill you). You will get a feel for how far away you can be and still knock them back. In truth, you shouldn't need to be anywhere near them.
When dueling a barb, it is easy to stay out of range of ww, you can't knock them back while they are doing it. When they see that ww is futile, they may try to attack you, and this is even more futile. I forced two high level ww barbs out of a duel game, because they could not kill me and I disrupted them so much they couldn't kill anything at all.
Paladin charge attack is damaging if you leap toward or around them. Leaping directly away works the best, getting you out of range and knocking them away again when you land.
You have nothing, and I mean nothing to fear from melee minions. You knock them around like they're common monsters. They should never get close enough to hit you. Discovered today that clay golem (possibly other golems too) are UNEFFECTED by leap
Basically, you are almost immortal against melee. Guided arrow is your worst enemy.
Hammerdins make for very boring duelers. It is easy to avoid their hammers with leap, and they don't care when you knock them around, so it is basically a stalemate. Neither one of you will be doing anything note worthy.