Ye Olde Inn

The card from the Game System and First Light has a different wording but generally the overall mechanic is the same.
What the card DOES NOT say, however, is when to apply the armor piercing ability. Considering there is no specifications you could probably do it whenever you want, but the 3 options are:
1.) the hero calls out BEFORE rolling the attack dice that he'd like to use the armor piercing ability, which gives him a 50/50 shot of landing a hit.
2.) Roll the attack dice first and when satisfied with the , apply to once per game ability, preventing the monster from rolling defense.
3.) After successfully getting his , the monster rolls defense, if the monster rolls a , could the hero, at this point, negate the defense?

Only the First Light card says "render your target unable to roll Defend dice." While the Game System one reads "your target may not defend himself."
The most likely of the 3 choices is Option 2, how does everyone else play it?

You could argue (2) is fine provided the declare it before a defense roll. But (1) is probably the intention.

Yes, we play 1. This is an effective weapon if given to the wizard along with a potion of strength. He can nip in at the end to finish off a powerful monster if the others have failed and he is out of spells or wants to save them.

The Druid had acquired this weapon in First Light. Working in a new 5-Man team in the Frozen Horror (Barbarian, Rogue, Elf and Wizard), she passed it onto the Rogue, who is a Weapons Expert with a Dagger, coupled with Opportunistic Striker, was used very recently to finish off a Polar Warbear.