If I flip opponent's flip effect monster by attacting it, can i chain fiendish chain to negate flip eff?

No you can't activate cards during the damage step unless they are ciunter trap cards or specifically state they can be used in the damage step

Or if they change the ATK/DEF of monster(s) on the field.

Fiendish Chain doesn't fill any of these requirements, so it can't be activated to negate a flip effect that was activated during the Damage Step.

Solved and Locked