As far as Dex builds go if you're looking to duel I'd recommend you go the Dex/Pyro route.
This build is just about as generic as it gets, but it's generic for a good reason. It's incredibly effective. The Chaos Blade and Silver Knight Spear should be the main weapons for this build, and the Estoc, Gold Tracer, Furysword, Demon's Spear, Uchigatana/Iaito, Falchion, Balder Side Sword, and Rapier are good backup weapons (these should be kept near the Chaos Blade in your menu). As far as backup rings go you may want to swap out the Dark Wood Grain Ring for Wolf or Hornet in some situations (or Red Tear if you're that kind of person). It'd also be a good idea to swap out the Mask of the Mother for the Mask of the Child (increased stamina regen) or the Crown of Dusk (increased pyromancy damage) after taking some damage. One thing that makes this build very powerful is the utility having an offhand pyro glove along with a mainhand kat gives you. 45 dex black flame deals good damage, breaks guards with ease, and does high poise damage. This high poise damage is very helpful since black flame can combo into a katana swing. Also, having an offhand pyro glove is very useful for the wakeup game. You can use the pyro glove to scare your opponent into rolling on wakeup, leaving themselves wide open for a chain backstab.
This build uses a buffed washing pole, which is one of the most fearsome buffed weapons in the game due to its long reach, bleed (which generally isn't used to actually bleed people but to force them to do something other than block, giving you an opening to capitalize on) and just being a katana (an excellent class of weapons). Wrath of the Gods is a decent spell, though careless use of it will leave you wide open for a roll backstab. If you land it with 50 faith though (especially while an opponent is rolling) you will deal massive damage, and put your opponent in a stunned state which gives you the opportunity to land a blow with your weapon or go for a backstab. The most challenging things about using this build are the low stamina, and the fact it's a bit on the fragile side (though that doesn't matter very much considering the fact it does tons of damage).
Also, I'll be talking about some of the general guidelines for a dex build here. First of all, 40 dex is as high as you want to go unless you want increased casting speed on pyro (which is very useful) or sorcery (not quite as useful as with pyro). In this case you'll want to raise your dex to 45. For vitality it's generally a good idea to get at least 45 (though I usually give my builds at least 50). Endurance should usually be raised above 30, and you should generally want to get close to 40, though going past 40 isn't the best idea unless you have a good reason since your stamina stops increasing past that point completely. For dex scaling weapons you'll generally want to get just enough strength to meet the base requirement for your main weapon, though some weapons like the Washing Pole have decent strength scaling as well, so in this case it could be a good idea to go with a "quality" stat setup (27/28 or 40 str with 40/45 dex) to get the most damage you can out of it. 16 str is usually the most you'll need since it gives you access to some of the best weapons in the game (Chaos Blade, Silver Knight Spear) and the Black Knight Shield, which works extremely well with the Estoc.