Would you envision Lack of Weakness as meaning an opponent wouldn't get any numerical bonus as well?  That would be kind of a cool power, in that the person moves in such a way that they defy study (always do things differently, or the like).

A few characters recently (I'm thinking of one from Darren Tenor's recent Zodiac) have the heightened Immunity.  Being as we often have poisons attack as Chemical, and being that Willpower does nothing to reduce that attack (on the combat chart), I'd be more inclined to have the character simply be completely immune to poisons and toxins.  It's a rare enough attack form that it still ends up being sort of a wimpy power.

Ironically, a year ago at GuardianCon one new 1st Level PC called Contagion used his poison attack for the first time against the character I mentioned above (Scorpio, from "Signs of the Zodiac", who is listed as a "Toxivore").  He was pretty frustrated in that she simply ignored his main attack.  The player didn't really "get" how rare of an ability that is, and it was a freak occurence that his first villain faced happened to have it! 

Body Power: Acid Touch: The character's body generates a highly corrosive acid that is released though their hands. Attacks as HTH. 2d8 damage. Victim/target is affected as though they were hit with a disintegration ray attack.