Can you defy a Piercing Light repeated?

Yes, Defy can counter a Piercing Light that has been repeated.

According to the RiftJudge FAQ #9196 and #2163, Defy and similar counter effects check only the printed (base) cost on the card, not the total cost paid (which includes any [Repeat] costs). Since Piercing Light has a printed cost of 2 energy and 1 power, it falls within Defy's targeting restriction (4 energy or less, 1 power or less).

Sequence of Events

1. 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.
2. Response: The opponent may respond with Defy.
3. Resolution: If Defy resolves, the entire spell—including all repeated effects—is countered and placed in the Trash. As per Rule 425.1.c, no costs paid (including the [Repeat] cost) are refunded.