Lladnar said:
Halfling Fighter 6 / Dualist 5
[..]
AC 30 = 10+7(dex)+4(int)+1(size)+1(monk)+3(wis)+4(bracers)
Also: +1 AC vs 1 opponent (dodge)
In 2 levels: fight defensivly -4 to hit, +10 AC
Rapier:
Hit: 23 = 11(bab)+7(dex)+3(enh)+1(feat)+1(size)
Critical: 15-20/x2
Damage: 1d4+2(str)+2(spec)+3(enh)+1d6(precise)
I assume you're suggesting Fighter 4/Monk 2/Duelist 5, to get the Monk AC bonus. Note that in 3.5E the AC bonus from monk will be Wisdom bonus only at this level. Taking a Halfling for the small size and maxing out Dex is good for AC. Power Attack might come in handy too actually.
That makes it AC 29.
The huge Elaborate Parry AC bonus does make a 7th level Duelist good fighting defensively, and an ok option. However, they are terrible before then, and for an 11th level character not good enough in the meantime.
With similar magic items and feats, the build I gave would have total attack bonuses of +18/+13, which is much worse than Lladnar's build. However, you could use a Greatsword, and your damage would be:
2d6+5(str)+2(spec)+3(enh) = 17 on average per hit, vs. 13 on average per hit with Lladynar's build (9.5 vs. creatures not affected by criticals).
Halfling Rogue/Duelist is probably an okay option for a secondary combatant, and you definitely need a feat that allows you to Sneak Attack undead as sithramir said. Though you'll have worse HP and saves than my build or the Unfettered build. [Side note: the major benefit of using a Rapier is to extend your critical range, try to keep a high attack bonus, and then throw in as much sneak attack, precise attack and other bonuses as you can.]
I think Storminator's Unfettered build is far better than all the Duelist builds. And you have more choice in how you spend your feats. Bladesinger isn't too bad either, but Unfettered is far closer to holding your own as a frontline fighter.