If I challenge a non-mighty Fiora, and the opponent then plays a card giving her +3 (making her mighty), do I need to pay the deflect cost?

Ruling: No, you do not need to pay the deflect cost. Fiora was already targeted when the challenge was declared.

Sequence:

• Challenge is declared targeting Fiora (who is not mighty at this time)
• Opponent plays a card giving Fiora +3, making her mighty
• The challenge proceeds without requiring deflect payment

Nuances:

• Deflect cost is only paid when you would put the spell on the chain, not when stats change after targeting