Does Defy stop Piercing Light repeated?

Yes, Defy can counter a repeated Piercing Light.

According to the card text and notes for both cards:

1. Defy's Targeting Restriction: Defy can counter a spell that costs no more than 4 energy and no more than 1 power.
2. Piercing Light's Printed Cost: Piercing Light has a printed (base) cost of 2 energy and 1 power.
3. Cost Checking: As established in the RiftJudge FAQ #2163 and #7995, Defy and similar counter effects check only the printed cost on the card, not the total cost paid (which includes any [Repeat] costs).

Because Piercing Light's printed cost (2 energy, 1 power) is within Defy's targeting range (4 energy or less, 1 power or less), Defy is a valid response to a Piercing Light, even if the [Repeat] cost has been paid.

Sequence of Events

• Finalization: You declare Piercing Light and choose to pay the [Repeat] cost. You pay the total cost (base + repeat). The spell is now finalized on the chain.
• Response: The opponent may respond with Defy.
• Resolution: If Defy resolves, the entire spell—including all repeated effects—is countered and placed in the Trash. As per Rule 412.1.c, no costs paid (including the [Repeat] cost) are refunded.