Scale Peeler

Once per Turn Action -

: Attack. Go again

Piercing 1

When this hits a hero, the next time they defend with 1 or more equipment this turn, those equipment get -1

while defending.

Rulings

From the Outsiders release notes

If two or more equipment are declared as defending cards at once, they would all get -1.
If a hero is hit two or more times by Scale Peeler and has not yet defended, the effects stack and the next time they defend with 1 or more equipment cards this turn, those cards have -1 for each time they were hit by a Scale Peeler this turn.
When the combat chain closes and effects like Temper trigger, the -1 would still apply when calculating the defense of the equipment.

Keyword notes

Piercing X - If this is defended by an equipment, it has +X{p}.

Piercing X is a static ability that generates a continuous effect.
If an attack with Piercing is defended by multiple equipment, it does not gain +X for each defending equipment.
If an attack has multiple instances of Piercing X, each effect is evaluated and applied separately.
If an attack with Piercing is no longer defended by an equipment, it loses its +X bonus.