If Fiora is buffed to 5 might with a reaction spell after an enemy has already targeted her with a spell, does the enemy have to pay for deflect, or is it too late?

Ruling: It is too late. The enemy does not have to pay for deflect because costs are paid at finalization, which occurs before Fiora receives the mighty bonuses from the reaction spell.

Sequence:

• Enemy targets Fiora with a spell
• Costs are paid at finalization (before reaction)
• Fiora is buffed with reaction spell after finalization
• The buff comes too late to trigger deflect cost