Negative Ac build?

[PhoenixRivers]

Ronin has banzai charge at level 2.

The start would be:
Barbarian 2 / Fighter 4 / Ronin 2

That would let you (with shock trooper), on a charge, let you take a penalty to AC that's double your BAB. With a 2 handed weapon, PA is 1:2, and Banzai charge is 1:1, so you're up to 1:3. Add in Leap attack, and you're at 1:5.

The build's level 8, so that would be an AC penalty of 16.
PA would net 1:4 on its 8 point penalty, for +32 damage.
Banzai Charge would net 1:1, for another +8.

Add on Frenzied Berserker to the end of this?

Barbarian 2/Fighter 4/Ronin 2/Frenzied Berserker 10

By level 13, you'd be getting -26 to AC, and you'd have the following bonuses:
PA (1:6): +78 damage
Ronin (1:1): +13 damage

By level 18? -36 to AC, and the following:
PA (1:8): +144 damage
Ronin (1:1): +18 damage

Barbarian gives easy access to pounce, and with the Wrathful Healing enhancement, we're looking at the following healing from bonus damage:

{table=header]Level | Damage | +Healing per hit | AC Penalty
8 | 40 | +20 | -16
13 | 91 | +45 | -26
18 | 162 | +81 | -36[/table]

Note: Chances are, two of your attacks are gonna be good, as well as any Robilar's AoO's. As whatever you're attacking is likely dead? The healing would be useful if you can find a way to get a charge when something's close enough to hit you. I like Travel Devotion myself, but belt of battle can do it, as can a number of swift action items that grant move actions.

The AC penalties will overshadow your AC at even mid levels, as long as you don't focus on it. I like Pierce Magical Concealment + Ring of Blinking for builds like this.