If I stick AP on a character with poison, say V Human Torch, does that mean if I based a fig with tough / invulnerability or Imperv it the poison would still cause a click of damage ?

Yes, It even works with Shake Off because it's damage dealt. Not necessarily attacks.

Does it work with damage shield?

Yes, because Damage Shield deals damage. AP and DS together is *almost* as good as Mystics (it still only works on Close Combat Attacks).

I played an all Hydra 500 point team 2 weeks ago consisting of Red Skull (first time ever playing him) w/ AP, Madame Hydra w/ Trick Shot and AP, and a bunch of vet agents, medics, sentinel troopers (Hydra stand-ins) and the like.

Red Skull was one of the MVP's! The Mastermind with the poison made it almost impossible to make any damage stick to him, and when I used him late-game to base a grouping of opposing figures I was dealing between 4-5 clicks of damage in a turn across several figures.

I won the tournament, and I had to face off against 2 different teams that had General Zod on them. Armor Piercing is most certianly my favorite feat, because it is so versitle.