How does Puncturing Light work against a creature buffed in size?
If its power is greater than 3 (whether its power has been increased by a "buff" or not), then it can't be targeted by Puncturing Light, so you are unable to cast Puncturing Light targeting it.

If the power of a creature targeted by Puncturing Light increases so that the creature's power is greater than 3 when Puncturing Light would resolve, Puncturing Light will be countered by the game rules (for having only illegal targets) instead of resolving.