If it throws up armor plated arms or something to block its vulnerabilities (fighting defensively), then yeah. That sounds like a fine way to represent that.

I think DR serves that purpose. You need to find the right weapon or material to get through the defenses.

yeah but paka is talking about more of a situational thing. why not give it a spell like ability similar to invisibility that protects it until it attacks?

Shock Trooper from Complete Warrior.

Basically you Power Attack, but instead of taking a penalty on attack you take a penalty on AC.

So I imagine a guy with a huge AC (so you can't hit him normally), but can Shock Trooper PA for massive damage. Except that lowers his AC by 10-20 points, so now you can cream him.

Mix AC with DR in order to reach the proper balance. Indeed, a version of Shock Trooper where you lose 1pt of DR to get +1dmg with PA (or +2 with a two-hander) would work just as well.

Note that DR 1/- is better than +1 AC when you're taking less than 20dmg a hit, equal when you are taking 20dmg a hit, and worse when you are taking more than 20dmg a hit.

Thanks! This sounds like a great idea, and pretty logical given Combat Expertise, but I don't think it's in Complete Warrior; might it be in a different supplement?

I'm pretty sure it's in CW. It's a Tactical Feat.

Yup, from the Feat Index:

Shock Trooper CW 112 You are adept at breaking up formations of soldiers when you rush into battle.

D'oh! Found it, thanks. It's not exactly what I want - but it does fit another encounter I really want to run.

Conditional DR.

Say, "In any round that ____ takes a complete defense action, ____ receives DR 40/-." That way, the really heavy hits are still do some damage, but characters with a really high attack modifier don't easily overcome the obstacle.