Switching gears a little;
One staple of video arcade games is the invulnerable bad guy with a soft spot. He's totally invulnerable until he "opens up," usually to attack, and at that point you can beat on him. The "Devastator" heavy warjack
in Warmachine seems to be built along that sort of philosophy.
Is there any particular way you'd do something like that in 3.5? I mean, one solution might be something with an extra bonus for armor while fighting defensively (say +10 natural AC versus a +4 natural AC), or something that could get a smaller bonus while overrunning opponents.