Let's compare one of the other other high AC builds to a Duelist build, and then compare that to a hybrid of the two.
It's worth noting that the other classes most commonly associated with Duelist builds also add Dodge AC. These classes include, but are not limited to: Swashbuckler, Whirling Dervish, Thief Acrobat. For this reason, I've not added them into the Duelist build, since we're mainly concerned about the total achievable AC here.
It is also worth noting that the bonus AC from Duelist' Elaborate Parry will only apply when using either the Fight Defensively or Parry modes. And that this comparison will be using the pre-nerf variant of Elaborate Parry.
The numbers below are purely based on a non-buffed state. Magic/UMD changes any build, and should not be a concern when balancing classes and feats. Otherwise magic/UMD becomes mandatory for all builds, which is bad for balancing too.
I have opted for maximizing Dexterity on the Duelist build, rather than Intelligence. However they are interchangeable for the purposes of AC.
Finally I will mention that this is taking the AC to the extreme, which includes spending all 5 (five) epic feats on Armor Skin, as well as grabbing Luck of Heroes at level 1.
Classes |
20 Man-at-Arms
10 Dwarven Defender
|
20 Fighter
10 Duelist
|
20 Man-at-Arms
10 Duelist
|
Total AC |
73
|
68
|
75
|
Armor AC |
8 Entropium Full Plate
2 Heavy Armor Optimization
3 Acrobatics
1 Luck of Heroes
4 Enhancement
|
3 Acrobatics
1 Luck of Heroes
4 Enhancement
|
8 Entropium Full Plate
2 Heavy Armor Optimization
3 Acrobatics
1 Luck of Heroes
4 Enhancement
|
Deflection AC |
4 Enhancement
|
4 Enhancement
|
4 Enhancement
|
Dodge AC |
5 Combat Expertise
4 Improved Defense (DD)
4 Enhancement
3 Athkatlan Triparite
2 Fight Defensively
|
10 Elaborate Parry
1 Dodge
4 Enhancement
3 Athkatlan Triparite
2 Fight Defensively
|
10 Elaborate Parry
1 Dodge
4 Enhancement
3 Athkatlan Triparite
2 Fight Defensively
|
Natural AC |
10 Base
5 Armor Skin
4 Amulet
|
10 Base
5 Armor Skin
4 Amulet
|
10 Base
5 Armor Skin
4 Amulet
|
Shield AC |
4 Tower Shield
2 Shield Block
1 Shield Specialization (Tower)
4 Enhancement
|
N/A
|
4 Tower Shield
2 Shield Block
1 Shield Specialization (Tower)
4 Enhancement
|
Other AC |
3 Dexterity Bonus
|
10 Dexterity Bonus
7 Canny Defense
|
3 Dexterity Bonus
|
Attribute Requirements |
13 Base Dexterity
16 Total Dexterity
13 Base Intelligence
|
20 Base Dexterity
30 Total Dexterity
20 Base Intelligence
24 Total Intelligence
|
13 Base Dexterity
16 Total Dexterity
13 Base Intelligence
|
Attribute Increases (from levels) |
Whatever you want
|
6 Dexterity
1 Intelligence
|
Whatever you want
|
Alignment Requirements |
Lawful
|
None
|
Lawful
|
Racial Requirements |
Dwarf
|
Tiefling (+2 Dex, +2 Int, [-2 Cha])
Drow (+2 Dex, +2 Int, [+2 Cha])
Air Genasi (+2 Dex, +2 Int, [-2 Wis, -2 Cha])
|
None
|
Required Feats |
Armor Skin (Epic Feat) x 5
Athkatlan Triparite
Background: Acrobat
Combat Expertise
Dodge
Luck of Heroes
Shield Specialization (Tower)
Toughness
|
Armor Skin (Epic Feat) x 5
Athkatlan Triparite
Combat Expertise
Dodge
Luck of Heroes
Mobility
Weapon Finesse
|
Armor Skin (Epic Feat) x 5
Athkatlan Triparite
Combat Expertise
Dodge
Luck of Heroes
Mobility
Shield Specialization (Tower)
Weapon Finesse
|
Based on the above, we can see that the Duelist build is actually the lowest in AC of the three builds, despite getting a lot of "free" AC from Elaborate Parry.
When adjusting for the post-nerf of Elaborate Parry, both the Duelist and the hybrid build suffer in AC, while the other typical tanky build remain unaffected. In fact the hybrid build is still better off on AC post-nerf than the Duelist was pre-nerf.
Stepping outside the AC discussion for a moment, the hybrid build isn't using Combat Expertise, and could therefore pick up Power Attack instead. This will put it at the same AB as the other tanky build, but with superior AC (+2) and Damage (+5). And this is, I believe, where the imbalance presents itself.
Additionally the total AC of the Duelist build will drop by 1 or 2 points depending on what race is chosen other than the three listed ones. This is before considering putting points into Str, Con, Wis, or Cha, which will further reduce the total AC provided by Duelist, as points for those increases have to be taken out of Dex or Int.
Back to the discussion on AC.
Duelist isn't the highest AC class out there. Very few of the other classes, which provide additional AC, do so through non-Dodge AC. Those few that do, either require armor or a shield, or have difficult to achieve requirements creating an anti-synergy with Duelist — yes it's possible to combine the two, but the build will suffer immensely, while only being good at defense. As established in the prelude, anything, which only adds Dodge AC, will yield the same total AC.
Edit:
For anyone considering what buffs will add to this, the answer is that it won't change the results.
Tortoise Shell adds +5 Natural AC to the total
Improved Mage Armor adds +2 Armor AC to the total
Shield of Faith adds +1 Deflection AC to the total
That's +8 Total AC for all three variants.
Both the tanky build, and the hybrid one will get an additional +1 Shield AC from Magic Vestments.
The tanky build will get an additional +2 Dodge AC from Inspire Defense/Recitation before reaching the Dodge Cap of 20. The others are already capped.
This brings the totals to 84 for the tanky build, 76 for the Duelist build, and 84 for the hybrid build.
With buffs on the Duelist build is then 8 behind in AC to either of the other builds, where previously it was only 5 behind the tanky build, and 7 behind the hybrid build.
TL;DR: Buffs puts Duelist even further behind, while bringing the tanky and hybrid builds to even ground.
Personal opinion
The nerf to Elaborate Parry hurts the unarmored Duelist more than it does the hybrid builds using armor. Even with the pre-nerf Elaborate Parry an unarmored Duelist is not even the highest AC build available.
Considering the investment required to get the AC benefits from Duelist for an unarmored build (high Int, high Dex), which is faced by neither of the two other options, it seems fair that the unarmored Duelist maintain its high bonus AC from Elaborate Parry.
If the intention is to balance the hybrids, such that they must choose between high AC, AB, or Damage, rather than get benefits in all three, then the better option is to disallow armored builds to benefit from Elaborate Parry at all. This due to the post-nerf hybrid still having superior AC to the pre-nerf Duelist. What a hybrid build will then get out of including those Duelist levels is the thematic aspect and class fantasy to go along with Improved Reaction (Self Haste) and Grace (+2 Reflex Saves).