Slattern's "Unstoppable" ability reads:

"Slattern can use invulnerability. After damage is reduced Slattern ignores all but one damage dealt to it."

Does P/PB and EW still do penetrating damage to Slattern?

Yes and no...P/PB would ignore the invulnerability and then he'd ignore all but 1 of that damage.

You're correct, but let's not get our Heroclix game terms confused.

Invulnerability would not REDUCE the damage dealt by an attack using P/PB or EW.

With Slatterns power, you calculate how much damage was reduced (whether it was reduced or not does not matter), THEN you ignore all but 1 of the damage that woul be taken.

Thank you.