Welcome to the Official Redemption® Message Board!
If we decide that you can choose to discard any card from battle (even an opponent's card) does that also mean you can discard a card from any hand? It seems like the condition should be the same for both options since no condition is stated.
nope. you proved that he is awesome.
If Cain is targeted by an opponent's special ability, that player must discard a card from hand or (a card they control in) battle.