Q: Can I keep Superhuman Strength in play if the attacked enemy is already stunned?

A: No, you must discard Superhuman Strength after She-Hulk makes a basic attack regardless of whether or not the villain is already stunned. -(Developer Ruling)

Q: Does Superhuman Strength discard after only using a basic attack or any Attack event?

A: You can play an Attack event to trigger Superhuman Strength and stun the attacked enemy. While it can create a situation where you miss out on the +2 ATK, that is ultimately the player’s choice. -(Developer Ruling, Hall of Heroes)

This card stays in play until you use an attack action with She-Hulk. At the moment, I think the only card that takes advantage of this is Counter-Punch (would do 5 damage and not discard Superhuman Strength).