Shade
Monster Junkie
Anyone else notice how they wanted to make superzombies but just added HD and anti-abilities? It would be much cooler just to have a few HD and have them be tough. Oh well.
I think they were trying to simulate the "falls apart, but keeps on attacking" schtick. You know, the ol' one-armed torso still crawling after the protaganist cliche in horror and sci-fi flicks?

Back to the task at hand. I could go with 1/3 or 2/3 hp. As an alternative, each hit could add a penalty with a certain % chance. It's more "swingy" and less "grindy" but might be easier. Maybe 30% of hits?
That might make 'em too tough, though. This is one of those unfortunate monster concepts that doesn't really fit well within the ruleset.
Another options just came to mind: multiple stat blocks. We could start with the 6 HD "super zombie" with two slams and a bite attack, then after it is "destroyed", switch to the "damaged" stat block, which has lesser HD, Str, and maybe loses a slam attack. Finally, we'd have the "heavily damaged" zombie, with only a bite left and only half its movement rate. It's a bit more work for us, but would make things easier on DMs.