[Humabout]

Might be able to use Vulnerability with some mods on it. Alternatively, use Extra HP, DR, IT (DR) with a minor accessability-ish limitation. Otherwise, I'm not entirely certain.

Personally, if PC isn't playing it, I'd not bother statting it beyond the penalty on the targeted attack and any damage requirements/limitations on the weak spot. I.e. a Regenerator has five weak spots that are only visible wtih thermal imaging. Destroying these points will prevent it from regenerating anymore. So give the weak spots X HP and a -Y hit location penalty. And you can only target them specifically if you can see IR.