In my opinion, the prestige class Duelist (from the SRD) seems a bit weak, although the concept is interesting, as some of the abilities.
Agreed on all points, with the exception of “bit” – the duelist is one of the weakest options in core.
The problem for the duelist is this: the einhander style it is based around is awful. There are basically three approaches to core melee:
Damage multipliers from two-handing. Using both hands results in 1½×Str to damage, and 2:1 returns on Power Attack. Cannot be done einhander.
Damage bonuses and large numbers of attacks. Sneak Attack and others apply a damage bonus on each attack, so they work best when you hhave lots of attacks: that basically means Two-Weapon Fighitng (or Rapid Shot). Cannot be done einhander.
Reach, attacks of opportunity, and tripping. Tripping is moderately effective battlefield control, and reach allows you to apply it to a reasonably large chunk of the battlefield. Cannot be done einhander (the only one-handed reach weapon is the whip, which does not threaten).
In short, the einhander fighting style is a massive drawback, and nothing the duelist gains comes even remotely close to justifying it. The best way to play a duelist is to ignore Precise Strike entirely, and just go ahead and use a two-hander with Power Attack. At least Acrobatic Charge is marginally useful.
Or you can try the trick of using natural weapons, arguing that they aren’t a “weapon in her other hand” as Precise Strike words the limitation. Certainly druid and lycanthrope are fairly effective.
But neither approach really screams duelist to me. That problem has even come up here before: we have a question seeking duelist replacements that may interest you (even though the question itself did not really define well what it was looking for and probably should have been closed and narrowed down).
All that said, a challenge is a challenge! Core does not offer a lot for the duelist, and I cannot promise that this will be particularly effective, some some thoughts on a duelist who actually fights with a single one-handed or light weapon.
Concept: Extra Attacks without TWF
We want to get more attacks to apply Precise Strike, pitiful as it is, more often.
Whirling Frenzy Barbarian 2
This is your first source of an extra attack. Whirling frenzy replaces rage and gives you an extra attack, along with some other stuff. Barbarian is also a big, strong class, with a nice big HD.
The second level gets you uncanny dodge, which is nice-ish? Mostly, it’s more HP and more BAB, which you need. Using the Wolf Totem variant can get you Improved Trip instead of uncanny dodge, which may be the better deal.
Cobra Strike Monk 2
Monk gives you flurry of blows, which is another extra attack. Down to a −4 penalty now, which is bad, but you need the attacks. The Cobra Strike fighting style gives Dodge and Mobility as bonus feats, which you need way more than anything else monk could give you. Evasion is tacked on so yay.
You also get Wisdom to AC when unarmored. Since you already had to be unarmored for canny defense, that’s fairly nice, but you really shouldn’t have especially high Int or Wis. But at higher levels a +2 or +4 enhancement bonus to each is much cheaper than a +4 to one and actually attainable, unlike +8 to one, so that’s something.
Alignment is awkward here: you’d really rather start as a barbarian to maximize the d12 HD over the monk’s d8, but you can’t be Lawful while retaining rage (and thus whirling frenzy). You can stop being Lawful and keep flurry of blows, however.
This is ultimately not actually that hard to justify: a barbarian savage for whatever reason (probably cuz he got beat in combat) joins a monastery, tries to be Lawful and learn their ways. Sticks with it long enough to learn some things, but ultimately can’t keep to that lifestyle and leaves or is kicked out. Result: Barbarian 1/Ex-monk 2 who is Neutral with respect to Law and Chaos.
You can now attack three times with a single (monk) weapon during a whirling frenzy. You only get one of those per day; unfortunately, getting one more use per day for every four levels of barbarian is really slow going.
Psychic Warrior 4+
Psychic warrior gets you bonus feats, which are useful, and also gets you psionic powers, which are more useful. Your BAB suffers, which is quite painful, but getting expansion and psionic lion’s charge is a really big win for you.
Especially psionic lion’s charge. Like other answers, pounce is really important here. You will have painfully few power points, but a few times per day you’ll be able to slam into someone across the room and tear into them pretty well.
Note that bite of the wolf and claws of the beast could be used similarly to the tiger-shaping of other answers. If you’re going that route, though, the other answers are much better at it. The inertial armor and vigor powers may be more appropriate, offering some defensive ability. Unfortunately, all of these things are competing for your extremely limited power points.
After completing duelist, psychic warrior is where you want the rest of your levels. You could go for slayer or war mind, to get full-BAB manifesting, but the missed level or restarting your progression doesn’t seem worth it.
A side-note about the RAW of psionic lion’s charge: it is worded very poorly. RAW, its Instantaneous duration means that its effect is to change reality in some real way, and then the power ends. The effect, in the case of psionic lion’s charge, is “You gain the powerful charging ability of a lion. When you charge, you can make a full attack in the same round.” RAW, that would be permanent. Normally, that’s overpowered, but here, well, the duelist needs all the help it can get; I’d allow it, anyway. The Augmentation is also problematic: it says “For every additional power point you spend, each of your attacks after a charge in the current round gains a circumstance bonus on damage equal to the number of additional points spent.” The problem here is that it says that each point gives a bonus equal to the number of points, so that the total bonus is equal to the number of points squared. Spending 8 power points on this power is prohibitively expensive for you, but a +25 bonus to attack and damage is still ridiculous. That one I probably would not allow.
This produces a weird einhander charger. It doesn’t work very well; it certainly keeps up with neither a real charger nor the shapeshifting builds of other answers. I don’t really recommend it. But it does seem like it is just about the best way to do the einhander thing.
For items, your weapon has to be a siangham; it’s the only monk weapon that works with duelist. You’re hitting a lot with it and your damage bonuses leave something to be desired, so pile those onto it.
Though you need to have Weapon Finesse, you get bonuses to Strength from whirling frenzy, and you still use Strength for damage. I think you should consider leaving Dexterity at marginal values (e.g. don’t dump it, but don’t pump it either), and focus on Strength for attack and damage.
You want gloves of dexterity, headband of intelligence, and periapt of wisdom. Getting +2 to each one costs 12,000 gp, less than +4 to any one of them would cost. An amulet of health may win out over the periapt of wisdom though. A belt of giant strength is, as mentioned above, probably even more valuable to you, though.
And of course, the most valuable cloak of resistance you can reasonably afford. Two levels of monk certainly help your saves a lot, but you should never neglect the cloak of resistance.
For other readers who go beyond the SRD, there are a few things that warrant mention:
Complete Adventurer, Swashbuckler—Weapon Finesse as a bonus feat at 1st, Int-to-damage at 3rd. Not worth any more levels than that, but it hits two high points for you. Build probably becomes Barbarian 1/Monk 1/Swashbuckler 3 as a psionic race so you take war mind, for Barbarian 1/Monk 1/Swashbuckler 3/War Mind 5/Duelist 10 in the end.
Complete Champion, Lion Spiritual Totem—Trade your fast movement for pounce. Obvious choice, makes psionics much more “nice to have” rather than “absolutely critical.”
Dragon Compendium, Kung Fu Genius—Use Intelligence instead of Wisdom for monk abilities. With canny defense, Int-to-AC twice.
Dungeonscape, Factotum—Int-to-nearly-everything. Just a good class in general. The build can look much like the swashbuckler build, but with factotum instead of swashbuckler.
Tome of Battle, sublime maneuvers—Great improvement over regular attacks, can greatly mitigate the deficiencies of an einhander style. The warblade class also puts Intelligence to moderate use, which is nice. If the DM rules that the swordsage class’s Wisdom-to-AC bonus is similar enough to the monk’s to work with Kung Fu Genius, that may be even better.