Most people would agree that the Prestige Classes (PrC) no longer shine so brightly compared to the base classes in Pathfinder. In 3.5 the most optimized builds usually had several different dips of various Prestige Classes. I think the main reason for this was that the base classes tended to be on the weak side, PrCs were front loaded with cool class abilities, and there was no incentive to stick with a class. At the end of the 3.5 era I think most people were sick of the vast number of Prestige Classes out there and so with the coming of Pathfinder, Prestige Classes were few and largely sucky compared to the beefed up base classes.
In the Pathfinder Beta, the Duelist prestige class was very strong defensively but weak offensively. The release version nerfed some of its defensive strength but did little to boost it on the offensive side. The poor Duelist has been left out in the cold sitting in a dark corner next to the Assassin PrC - things no PC in their right mind would take.
Over time more splat books come out offering more options and often things that suck alone synergize nicely with new options. Sometimes this is intentional and sometimes not. The Dervish Dance feat is specifically worded so it blends nicely with the Duelist PrC. This feat allows you to use dex for damage. It comes with lots of restrictions but the Duelist faces the same restrictions for most of his class skills so it doesn't matter.
Lastly there is the question of what classes to use for entry to Duelist? The Rogue seems good thematically but sucks in practice. Rogues have trouble hitting in combat and sneak attack looks much better than it actually is. My current preferred set up is fighter 5/monk2. We'll use the Lore Warden Archetype for the fighter classes. We trade away Medium, Heavy, and shield proficiency which we would not use anyway and get 2 extra skill points per level, Combat Expertise and a small boost to CMB/CMD. The fighter levels also gets us enough bonus feats to qualify for Duelist, Weapon Specialization and Weapon Training, which when combined with Gloves of Dueling give us a total of +3 to hit and +5 to damage. The 2 monk (Master of Many Styles archetype) levels are for style feats and Evasion since we'll have a really good Reflex save. If you started with Fighter 1/Monk2 you could have Crane Wing allowing you to deflect one attack per turn which is pretty strong when most things only have one attack at this level. The Crane Style feats work when fighting defensively which combines nicely with the Duelist's Elaborate Defence ability.
Here is my 10th level Duelist build. Fighting defensively, To Hit drops by one but AC goes up by 3. Combat Expertise can also be used to punch AC up into the stratosphere (AC 37).
Male Halfling Duelist 3 Fighter (Lore Warden) 5 Monk (Master of Many Styles) 2
LG Small Humanoid (Halfling)
Init +8; Senses Perception +18
AC 31, touch 27, flat-footed 20 (+3 armor, +9 Dex, +1 size, +1 natural, +2 deflection, +2 dodge)
hp 87 (8d10+2d8+20)
Fort +13, Ref +16, Will +12
Defensive Abilities Canny Defense +3, Evasion, Parry
Spd 30 ft.
Melee +2 Scimitar +20/+15 (1d4+14/18-20/x2) and
Unarmed Strike +16/+11 (1d4/20/x2)
Special Attacks Precise Strike, Weapon Training: Blades, Heavy
Str 10, Dex 20/22, Con 10/12, Int 14/16, Wis 14/16, Cha 10
Base Atk +9; CMB +10; CMD 36
Feats Combat Expertise +/-3, Crane Riposte, Crane Style, Crane Wing, Dervish Dance, Dodge, Improved Unarmed Strike, Mobility, Monk Weapon Proficiencies, Stunning Fist (4/day) (DC 18), Toughness +10, Weapon Finesse, Weapon Focus: Scimitar, Weapon Specialization: Scimitar
Skills Acrobatics +19, Disable Device +16, Fly +8, Knowledge (Dungeoneering) +16, Knowledge (Engineering) +16, Knowledge (Local) +14, Perception +18, Perform (Dance) +5, Stealth +23
Languages Common, Halfling
SQ AC Bonus +3, Enhanced Mobility (Ex), Fearless, Fuse Style (2 styles) (Ex), Stunning Fist (Stun) (Ex), Unarmed Strike (1d6)
Combat Gear +2 Scimitar; Other Gear Amulet of Natural Armor +1, Belt of Physical Might, DEX & CON +2, Bracers of Armor, +3, Cloak of Resistance, +3, Handy Haversack (empty), Headband of Mental Prowess, INT & WIS +2: Disable Device, Ring of Protection, +2
AC Bonus +3 The Monk adds his Wisdom bonus to AC and CMD, more at higher levels.
Canny Defense +3 (Ex) +INT bonus to AC (max Duelist level).
Combat Expertise +/-3 Bonus to AC in exchange for an equal penalty to attack.
Crane Riposte When you deflect an attack, you may make an attack of opportunity
Crane Style Take -2 penalty when fighting defensively
Crane Wing May deflect one attack per round while fighting defensively or using total defense
Dervish Dance Use Dex modifier instead of Str modifier with scimitar
Enhanced Mobility (Ex) +4 AC vs attacks of opportunity while moving out of a square.
Evasion (Ex) If you succeed at a Reflex save for half damage, you take none instead.
Fearless +2 morale bonus vs Fear saves.
Fuse Style (2 styles) (Ex) At 1st level, a master of many styles can fuse two of the styles he knows into a more perfect style. The master of many styles can have two style feat stances active at once. Starting a stance provided by a style feat is still a swift action, but whe
Improved Unarmed Strike Unarmed strikes don't cause attacks of opportunity, and can be lethal.
Mobility +4 to AC against some attacks of opportunity.
Parry (Ex) Forego an attack to defend against enemy attacks.
Precise Strike (Ex) Extra damage when using light / 1-handed Piercing weapons.
Stunning Fist (4/day) (DC 18) You can stun an opponent with an unarmed attack.
Stunning Fist (Stun) (Ex) At 1st level, the monk gains Stunning Fist as a bonus feat, even if he does not meet the prerequisites. At 4th level, and every 4 levels thereafter, the monk gains the ability to apply a new condition to the target of his Stunning Fist. This conditio
Unarmed Strike (1d6) The Monk does lethal damage with his unarmed strikes.
Weapon Training: Blades, Heavy +1 (Ex) +1 Attack, Damage, CMB, CMD with Heavy Blades
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