I attack a battlefield where my only attacker is a Poro. In the showdown my opponent plays a spell that would kill my Poro. I react by playing Stalking Wolf with Ambush and choose to kill my Poro as part of its cost. Is this legal?
It is not legal on the battlefield you are attacking. It is legal if Stalking Wolf is played to a different battlefield than the one where the Poro is. (Stalking Wolf may receive updated wording later.)