If you focus only on the core rules, as I prefer to do, you will not find much in the way of class features to boost your armor class. There are AC-booster spells, such as mage armor and magic vestment, but they tend to be replicable with magic items, such as bracers of armor and magic armor. However, there are three prestige classes in the Dungeon Masters Guide that boost AC. One of these is the Dragon Disciple, and it works fairly well for characters pursuing a high-dexterity-with-armor AC path.
I have ranted about the Dragon Disciple in the past, and so has the Cranky Gamer, but I have seen more splat book PrCs since then, such as the Thrall of Demogorgon in the July issue of Dragon Magazine, and I have softened my opposition to the Dragon Disciple a little more than somewhat. The requirements pretty much force you to take two levels in sorcerer or bard (or assassin), and then to become a combat oriented character rather than a caster, so that mitigates. My biggest beef is the Dragon Apotheosis ability at level 10, which is completely bogus. Replace that with a +1 AC and +2 charisma (keep the 60 feet blindsense). Just say no to the half-dragon template, breath weapon boost, +4 Strength, low-light vision, darkvision, immunity to sleep and paralysis effects, and immunity to the energy type used by his breath weapon. Now you've got yourself a balanced and playable PrC.
So having fixed the Dragon Disciple, how is it relevant to the discussion at hand? The prestige class gives a sweet +4 bonus to natural armor over the course of ten levels. That's not an enhancement bonus, either, so it stacks with the bonus from a barkskin spell or an amulet of natural armor. With that as a base, what races, classes, skills, feats, and magic items work best to make your dragon disciple tough to hit?
Races -- a small race gives a +1 size bonus to AC, so start there.
- Gnome: An uneven multiclass with bard -- bard4/paladin2 or bard4/cleric1 or bard 4/fighter1 -- works well for gnomes, who favor bards.
- Kobold: If you do want a sorcerer multiclass, a kobold has some advantages, including a +1 natural AC and a +2 bonus to dex. Sorcerer4/fighter1 and sorcerer4/cleric1 both give you the armor proficiency you need. You'll want to focus on spells with no somatic components to avoid spell failure. Also there is the aesthetic advantage that kobolds look like mini-dragons already.
- Halfling: Size bonus to AC, Dex bonus (also helps AC), sweet save bonuses ... halfling is an atractive option. A pure five-level bard build gives you the levels you need to take craft wand, which can really help get more mileage out of your spellcasting. Bard4/paladin4 is also a good option for divine grace and using dragon disciple spell slots to add to paladin casting (though to do it you have to start as a bard, then convert to LG and take paladin later ... so you pay the 20 percent experience point penalty when you are bard4/paladin1 and bard4/paladin2.)
Required Class -- You must have spontaeous arcane spell casting, so pick one of these...
- Bard: Class skills include tunble, knowledge arcana, and speak lang, plus other dragon disciple synergies like spot, diplomacy, and gather info. Plus you can wear armor. Nice fit.
- Sorcerer: You have to take tumble cross-class, if you want it (and you do). Plus your hit dice and saves suck, you have no proficiency in armor, and you have very few skill ranks. Not to mention spell failure in armor. Bleck. But if you do it, get the still spell feat and take still reduce person for an extra +2 AC.
- Assassin: Yes, assassins use spontaneous casting to cast arcane spells, so they qualify. They use intelligence rather than charisma to do so, by the way. You'll probably want to five, seven, or eight levels of assassin, to get third level spellls, fourth level spells, or hide in plain sight, respectively. But even a three level dip is worth it. Of course, this postpones taking dragon disciple, since a third level assassin is at least an 8th level character.
Other Classes -- If you do multiclass, there are a number of compelling options.
- Cleric: Luck domain lets you reroll hit dice if your DM is nice ... very sweet. Good saves, armor proficiency. Not bad for a kobold sorcerer4/cleric1.
- Paladin: Divine grace lets you apply your charisma bonus to saves, which is sweet. Paladin 4 lets you use dragon disciple slots to boost paladin spell casting. More Bless Weapons please!
- Fighter: A one or two level dip gives a sweet extra feat or two.
Feats and Skills -- There are only a few worth mentioning...
- Skills: The only skill that affects your AC, tumble is worth it, even cross class. Five ranks give you better bonuses when fighting defensively or using total defense.
- PHB Feats: Dodge and Mobility (for when you tumble skill fails you) are the ones to look for here. Combat Expertise is also a taker, if you can muster the 13 Int. Other useful feats: Weapon Finesse, Spring Attack, maybe Craft Wand if you have five levels in bard.
- Monster Manual feats: If you're a kobold, or once you become a dragon disciple, Improved Natural Armor from the Monster Manual is essential, and you can keep taking it!
Finally, what armor do you use? You want it light, so your bard casting does not suffer. You want mithril, since the max dexterity allowance is higher. So look for a mithril chain chirt and buckler at lower level, then graduate to mithril chain or mithril breastplate, and finally to celestial armor.
The end result -- Let's look at a sample character created with these guidelines and the elite array (15, 14, 13, 12, 10, 8). We'll sample at levels 5, 10, and 15.
Level five (9,000 gp): Halfling Bard2/Fighter2/Cleric1 with 18 Dex. Feats: Dodge, Mobility, Weapon Finesse, Combat Expertise. Gear: +1 mithril chain shirt, +1 mithril buckler, +1 ring protection, +1 amulet nat armor. Armor Class: 24 (25 dodging).
Level ten (49,000 gp): Halfling Bard2/Fighter2/Cleric1/DDisciple5 with 20 Dex. Feats: Dodge, Mobility, Weapon Finesse, Combat Expertise, Imp Nat Armorx2. Gear: +3 mithril breastplate, +3 mithril heavy shield, +2 ring protection, +2 amulet nat armor, +2 Gloves of Dex, +1 AC Ioun Stone. Armor Class: 38 (39 dodging).
Level fifteen (200,000 gp): Halfling Bard2/Fighter2/Cleric1/DDisciple10 with 26 Dex. Feats: Dodge, Mobility, Weapon Finesse, Combat Expertise, Imp Nat Armorx4. Gear: celestial armor, +5 mithril heavy shield, +5 ring protection, +5 amulet nat armor, +6 Gloves of Dex, +1 AC Ioun Stone. Armor Class: 53 (54 dodging).
Add in +5 for combat expertise and +3 for fighting defensively, and we're looking at AC62 at 15th level. Not too shabby.
This is part of an extended series on Survivability in Dungeons and Dragons.